Plugin can’t be activated [closed]

The root of the problem is that isset expects a variable to be passed to it. Also, I’m assuming that you expect the get_post_meta() to return a scalar value and not an array, so I added the third parameter set to true.

// 3rd param: $single. Value of true means get scalar value, not an array.    
$ytlink = get_post_meta( get_the_ID(), 'songbook_video_link', true );

// Will ensure that $ytlink is boolean false, and not an empty string.
$ytlink = ( ! empty( $ytlink ) ) ? $ytlink : false;