The featured image is using the same functionality as the ‘Posts’ or ‘Pages’ section. Are you able to select featured images in a post?
I am also experiencing this issue upon attempting to set the feature image. It looks to be isolated within this plugin because I am still able to select featured images in posts and pages. It may be this particular installation because I also have the plugin installed on another client site and haven’t had similar issues. Any thoughts on what it could be?
In your theme somewhere (most likely the functions.php) there may be a like like this:
add_theme_support( 'post-thumbnails', array( 'post', 'page' ) );
If there is, you will need to add the post type ‘testimonial’ to that list like this:
add_theme_support( 'post-thumbnails', array( 'post', 'page', 'testimonial' ) );
Note: there may be more items in that list, so simply add it to the end.
