iTunes will not Quit, and will not log in to the iTunes store

My iTunes is locked. It will not quit. It will not force quit. It keeps demanding my password to log in to the iTunes store, and as soon as I enter my password, the dialog box just reopens and demands the password again. I have reset my password, and I can log in to my AppleID and iTunes elsewhere... but iTunes is stuck in a loop.

I'm on a 2011 iMac running Sierra 10.12.6.All my software is up to date.

HELP