You need to authorize before trying to insert an event via the api.

In your request add Authorization header. 

Example:

    curl_setopt_array(
        $curl,
        array(
            CURLOPT_URL            => $url,
            CURLOPT_POSTFIELDS     => json_encode($post_data),
            CURLOPT_RETURNTRANSFER => true,
            CURLOPT_HTTPHEADER     => array(
                "Content-Type: application/json",
                "Authorization: $auth" // <--- This is the important line
            ),
            CURLOPT_POST           => true
        )
    );

$auth example:

$username     = 'admin'; 
$app_password = 'GOBU V20P avUe Qnzi vPV7 TwIg'; // @see https://youtu.be/e_thybKPKHc?si=8ZUNpV6a8Cu5Ke3Q&t=349
$auth         = 'Basic ' . base64_encode($username . ':' . $app_password);

where $username is your actual login name

For $app_password you need to go to Dashboard -> Users -> Your profile and scroll down to:

You should use this password and not the one you login with.

//set auth token for posting
$username     = 'admin';
$app_password = '**** **** **** **** **** **** ****';
$auth         = 'Basic ' . base64_encode($username . ':' . $app_password);
/**
 * Create new event
 */
$new_event = array(
    "title" => [
        "raw" => "My New Event"
    ],
    "excerpt" => [
        "raw" => "Event short description"
    ],
    "content" => [
        "raw" => "Event full description"
    ],
    "stec_cal" => 6, // the calendar id number
    "meta" => array(
        'start_date' => '2023-07-11T12:30',
        'end_date'   => '2023-07-11T12:30',
        'all_day'    => true
    )
);
// Merge defaults with new event
$new_event = array_replace_recursive($defaults, $new_event);
// Send new event to the api
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, "https://test.surreyunion.org/wp-json/stec/v5/events");
curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($new_event));
curl_setopt($ch, CURLOPT_HTTPHEADER, array(
    'Content-Type: application/json',
    'Accept: application/json',
    'Authorization: ' . $auth
));
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
$response = curl_exec($ch);
// ... do something with the response
echo $response;
curl_close($ch);

$auth was missing value and it wasn't correctly added in the header. It was added as a string and not as variable.

I'd suggest you to create e helper function that will handle image uploads.

Here's example. 

Assuming this is the event post data:

$post_data = array(
    "title" => [
        "raw" => "My New Event"
    ],
    "excerpt" => [
        "raw" => "Event short description"
    ],
    "content" => [
        "raw" => "Event full description"
    ],
    "stec_cal" => [603], // the calendar id number
    "status" => "publish",
    "meta" => array(
        'start_date' => '2023-12-31T12:30',
        'end_date'   => '2023-12-31T13:30',
        'all_day'    => true,
        'timezone'   => 'stec_cal_default' // default to calendar timezone
    ),
    
    "fn_upload_image" => array(
        "https://www.google.com/images/branding/googlelogo/2x/googlelogo_color_272x92dp.png"
    )
);

This array "fn_upload_image" here will hold your images that you want to attach to the event. 

It should contain array with image sources.

---

And here's the custom hook that will handle the upload and attaching the image to the event "images" meta data:

/**
 * Handler for 
 * 
 * $post_data = array(
 *     ...
 *     fn_upload_image = array( 'http://image1.jpg', 'http://image2.jpg' ... )
 *    ...
 * )
 */
add_action('rest_insert_stec_event', function ($post, $request, $creating) {
    $fn_upload_image = $request->get_param('fn_upload_image');
    $uploaded_images = array();
    if (!is_array($fn_upload_image) || empty($fn_upload_image)) {
        return;
    }
    if (!function_exists('wp_crop_image')) {
        include_once(ABSPATH . 'wp-admin/includes/image.php');
    }
    foreach ($fn_upload_image as $image_src) {
        // upload image to media library
        $upload   = wp_upload_bits(basename($image_src), null, file_get_contents($image_src));
        $basename = basename($image_src);
        $mimetype = wp_check_filetype($basename);
        if ($upload['error']) {
            continue;
        }
        $attachment = array(
            'post_title'     => sanitize_file_name($basename),
            'post_content'   => '',
            'post_status'    => 'inherit',
            'post_mime_type' => $mimetype['type']
        );
        $attach_id = wp_insert_attachment($attachment, $upload['file']);
        // Generate attachment metadata and update the attachment
        $attach_data = wp_generate_attachment_metadata($attach_id, $upload['file']);
        wp_update_attachment_metadata($attach_id, $attach_data);
        $uploaded_images[] = $attach_id;
    }
    foreach ($uploaded_images as $order => $image_id) {
        $sizes = array('full', 'thumbnail', 'medium', 'large');
        $data = array(
            'id'            => $image_id,
            'sizes'         => array(),
            'dimensions'    => array(),
            'order'         => $order
        );
        foreach ($sizes as $size) {
            $image = wp_get_attachment_image_src($image_id, $size);
            if (!$image) {
                return false;
            }
            $data['sizes'][$size] = $image[0];
            $data['dimensions'][$size] = array(
                'width'  => $image[1],
                'height' => $image[2],
                'ar'     => $image[1] / $image[2]
            );
        }
        update_post_meta($post->ID, 'images', $data);
    }
}, 10, 3);

---

Basically what this hook does is it downloads the image, uploads it to the wordpress library and inserts the necessary meta to the event "images" meta.

The image data structure that is stored in event "images" meta is like following:

Array
(
    [id] => 5469
    [sizes] => Array
        (
            [full] => http://localhost:8888/wp-content/uploads/2023/11/googlelogo_color_272x92dp-19.png
            [thumbnail] => http://localhost:8888/wp-content/uploads/2023/11/googlelogo_color_272x92dp-19-150x150.png
            [medium] => http://localhost:8888/wp-content/uploads/2023/11/googlelogo_color_272x92dp-19-300x101.png
            [large] => http://localhost:8888/wp-content/uploads/2023/11/googlelogo_color_272x92dp-19.png
        )
    [dimensions] => Array
        (
            [full] => Array
                (
                    [width] => 544
                    [height] => 184
                    [ar] => 2.9565217391304
                )
            [thumbnail] => Array
                (
                    [width] => 150
                    [height] => 150
                    [ar] => 1
                )
            [medium] => Array
                (
                    [width] => 300
                    [height] => 101
                    [ar] => 2.970297029703
                )
            [large] => Array
                (
                    [width] => 544
                    [height] => 184
                    [ar] => 2.9565217391304
                )
        )
    [order] => 0
)

You just need to specify the full size url. The other sizes are handled by WordPress during the upload process. The uploaded image will be used as an image for the media carousel yes.

You could also assign the image as a featured image.

Try to var dump the events ids to see if they are being saved:

add_action('init', function () {
    $events = get_posts(
        array(
            'fields'        => 'ids',
            'post_type'      => 'stec_event',
            'posts_per_page' => -1,
            'post_status'    => 'any'
        )
    );
    var_dump($events);
    exit;
});

If the events are there it is possible they are missing some meta data.

Check if they have read_permission and edit_permission meta values, if their post_status is "publish".

Do you see any errors in the error_log file? Make sure WP_DEBUG and WP_DEBUG_LOG are set to true in your wp-config.php file.

Try turning off all settings in Calendar Dashboard -> Settings -> Language and see if the events show up.

---

This php code will delete all events it finds.

$ids = get_posts(array(
    'fields'     => 'ids',
    'posts_per_page'   => -1,
    'post_type'  => 'stec_event'
));
if (is_array($ids)) {
    foreach ($ids as $id) {
        wp_delete_post($id, true);
    }
}