Error: Embed .mov file via ‘Add Media’ not working

The default supported video formats are:

mp4, m4v, webm, ogv, wmv, flv

This code adds the Embed Media Player support for the QuickTime video files.

/**
 * Add support for the QuickTime (.mov) video format.
 */
add_filter( 'wp_video_extensions', 
    function( $exts ) {
        $exts[] = 'mov';
        return $exts;
    }
);

Suggestion:

.mov is not a web-friendly video format – very proprietary, very large files, and not compressed.

Best to convert it into a .mp4 file and then you can embed it natively into WordPress without the need for any additional plugins.