Those are warnings, not errors. They are because you’re not including “Offers” or “Performers.”
This is for the Events scheme, and Google may or may not require those to be included for it to be valid (they may be optional).
The image isn’t supposed to “Display” you’re just telling it what the URL is for the Event. If Google chooses to display this on Google (when someone does a search for Events), they can use the URL to show it at that point.
In this case, it looks to be working properly.
Try wp_get_attachment_image_url() instead. The function you’re using returns an array, not the URL.