Got a new phone and can’t verify card in Monzo. Every time I try it says “sorry about this. Something’s gone wrong, bu we’re not sure what. Please try again in a few minutes.” And I have been doing this for a few days and it won’t work. I deleted my card and closed the app and opened it again and it says the same thing? I also try to retrieve my card number when the card was added and got similar messages! What’s wrong? Why can’t I add my card to Apple Pay?
Have you tried deleting the app & reinstalling it?
If it’s a new phone, you may need to reverify your identity as well to ensure it is you using the phone. Also delete any pending Monzo cards in Apple Pay & try again after deleting/reinstalling the app 
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Try adding via Apple Wallet instead.
I finally got in by reinstalling the app, verifying my identity with a short video and then using the app to verify my card in the Apple Pay wallet. Only Monzo gave me this problem. Other cards were fine.