There are 2 possible issues: icloud restore and incorrect login. Can you login to the web portal (https://web.monzo.com) using the correct email address?
Please also check the 4th section of this post:
There are 2 possible issues: icloud restore and incorrect login. Can you login to the web portal (https://web.monzo.com) using the correct email address?
Please also check the 4th section of this post: