"This item was not added to your iCloud Music Library because an error occurred"

This is why Apple Stores are overloaded. I have 254 items like this. No explanation, no hints on fixing it.

Low quality is expensive.

PS. In this particular case it looks like iTunes and iPhone supported .mp4 audio, but iCloud does not. Long ago I ripped these and the software did .mp4.

I found the on drive files and used quicktime player to export as .m4a — a lossless transform that strips out the some of the .mp4 wrapper. Then I deleted originals in iTunes and added these back in.