Couple of thoughts here 
Are you seeing any declines show up in your monzo app or does the trail stop with Google pay’s error message?
It’s weird that it says “this card can’t be used with Google Pay here” - if it was due to root or a mental safetynet failure, the app tends to say “Google Pay can’t be used because the device has been rooted or modified in some way” (or something similar) and it wouldn’t allow you to add cards or use any cards to pay 
I’d try either reinstalling the Google Pay app or just removing and re-adding your monzo card to GPay and ensuring you follow all the steps there 
If you’re still having no luck, definitely send us a message through the in-app chat and we’ll do what we can to figure out what’s happening 