The authentication error being returned may occur if there is a mismatch with the email address or if you have ever changed your email address on Monzo. Please try again and ensure you are entering the correct email address for your Monzo account .
Another less likely reason the authentication error code can come up is if you have ever changed your email address on Monzo. In this case, you would need to speak to Monzo to ensure your email address is updated.
Should the problem persist, please get in touch with us and we will do our best to help