I keep trying to add songs to a playlist, but will not be added

OK, so the problem is not the playlist. Actually that is good news.

Just a hypothesis, but the problem track may have slight corruption. Since it is working in your library, try converting it to a different format (e.g. to AAC if it is now an MP3), and see if the new file behaves better:

Convert a song to a different file format with the Apple Music app or iTunes for Windows – Apple Support (UK)

Holtville,

There is no documented limit on the length of a playlist, but not clear what is going on. You could experiment with splitting the list in two (and potentially put them into a single playlist folder so they can easily play together). Also check if there is anything unusual about the songs that won't add, especially if they are recent additions to your library.

1. Before adding to a playlist, make sure the song is in your library and will play correctly.
2. Make sure it is a regular playlist (note symbol) not a smart playlist (gear symbol).