Hi there!
If you are getting this message when trying to play certain songs in playlists, first ensure that the songs you are searching for and the songs in your playlist are the same version. Your playlists may have a saved version of these songs that is no longer available in the Apple Music catalog. If the songs in your playlists are a different version than the ones you locate when you search, then remove these songs from your playlist and add the version that plays to resolve this issue.
If the songs are the same versions and this continues, then next restart your iPhone and ensure it is fully updated. This refreshes the software of your device and installs bug fixes that can alleviate unexpected behavior like this. These articles show how to do this:
• Restart your iPhone
• Update your iPhone or iPad
Cheers!