iTunes Match subscribe button does nothing

I recently dropped Apple Music to stop paying £15/month to listen to one song I didn't have, and instead wanted to have my personal music library available for playing through our Sonos using an old iPad as a controller screen and Siri interface. Even though I am signed into the old iPad with my account the songs in my Music library on my Mac don't match the songs I can see on my old iPad. I figured this was because they weren't iTunes purchases but ripped from CDs (remember that?) and they aren't in a cloud. After a bit of searching I rediscovered iTunes Match - it's what I need (provisionally) to sync libraries across devices without Apple Music. However, when I press on the button in the iTunes Store to subscribe to iTunes Match, nothing happens.

If I sign out and then press the button, a sign in pop up comes, I sign in. Nothing more happens.

I'm fairly sure (because I can't see 'Sync Library' in my apps that I don't already have iTunes Match, and it's not letting me buy it...

Why can't I purchase an iTunes Match subscription...?