Deleting music from iPhone library

Deleting files on your iPhone does not automatically create more space in iCloud:

Manage your iCloud storage - Apple Support

What's the difference between device storage and iCloud storage? - Apple Support

You will have to archive data from iCloud directly, to gain some free space back:

Archive or make copies of the information you store in iCloud - Apple Support

Did you sync the music through iTunes on your computer, or are you using the Apple Music subscription?

If you used iTunes, connect it to your computer, deselect the music you don't want to have on your iPhone, then sync.

Did you already try to force restart your device?

Force restart iPhone - Apple Support