Can't add computer to iTunes Match

Hi, I can't add my Mac Mini with Catalina to iTunes Match. I get the same symptoms that have been reported by other people, i.e. you click "Add This Computer" and the text fades but nothing happens. If you quit Apple Music and try again you see the button ready to be clicked again. I do have an iTunes Match subscription. The other reports of this problem I see mainly seems to date to before iTunes was removed so I can't follow that advice. It used to work on this same computer but recent upgrade seems to have screwed it.

Basically I would like to subscribe to Apple Music but keep my iTunes Match settings, so I'm trying to get Match working again first.

Thanks,

David