Spoffler Try setting up Revolut on a non-GrapheneOS device. Then try setting it up in a new GrapheneOS profile while you have an active session in the non-GrapheneOS device. Perhaps they've blacklisted your account temporarily until you log in on a "trusted OS". That's the only thing that would make sense to me. If that fails you can try a factory reset as a last resort. If it still doesn't work after a factory reset then it might be some kind of a hardware UUID block and the only way to confirm this is to contact support and see if they'll give you any details on why you can't log in on your main phone.
I've just installed Revolut (from Aurora Store) on a freshly set up GrapheneOS device. It just asked me for my email, passcode and then facial recognition. So it currently works fine on both my new and old GrapheneOS devices (Revolut versions 10.122.1 and 10.115.2). The only thing I don't have is Passkey/Fingerprint unlock (I don't think it works on Graphene anyway? Since I'm getting an error trying to set it up.)
Now again, I also had this "unsupported device" message pop up on my older GrapheneOS device but the message got dismissed somehow after I updated my Revolut app. So it could be related to the version of your Revolut app.