Apple Music Playlist Strange Pop-up Message

As noted, the issue is likely to be that you attempted to add ineligible content to the playlist, which then makes the list ineligible.

Ineligible

Apple Music might not be able to upload the song because the song wasn't purchased from the iTunes Store, was purchased using a different Apple ID, or is too large of a file.

tt2

One presumes that ALL SONGS is the name of a regular playlist that currently only contains content that is eligible for iCloud Music Library, but that you're trying to add ineligible content to it. See Identify cloud status icons in your music library on your Mac or PC - Apple Support. If you proceed the playlist will be removed from your iCloud Music Library, not the songs themselves, and this will become the only computer you can access the list on.

tt2

You're welcome. 🙂

tt2