Have you tried logging out (and invalidating the access token) and re-starting again? I feel like the section at the top should show your account ID, and its absence makes me think you need to re-authenticate.
Hey Couple of things it could be… It could be you’re logging into the dev portal with an email that’s not your monzo account email, or it could be that you’re not substituting $account_id for your actual account id (the dev portal should do this, but might break sometimes)
I feel so stupid! It turns out I was. I originally tried logging in with the correct email address but I never recieved an email from Monzo so assumed it must have therefore been my other address which did trigger an email.