Gonna take a stab in the dark and guess that Monzo’s action without notice here to compensate for inefficiencies in their service has broken the wallet integration.
I had an inkling it might happen when I saw the post, but then no one mentioned anything until now.
Something Apple will need to fix in a future software update if this is the cause.
Edit: tested mine, and I can’t link it either. I get the same error as you for my current account. And Monzo flex throws up some weird API login screen at Monzo.