Try using the same email you signed up with. A few people have had this issue this week and each case has been they’re entering a different email to what was signed up with.
it doesn’t usually ask for your number. It emails you a magic link.
Double and triple check, because you are using the wrong email
As it doesn’t recognise that email, it thinks you’re new, which when it gets to the mobile number stage, it says there’s already account with that number.
Was your old phone an iPhone and is your new phone also an iPhone and have you restored a backup from your old phone to your new phone?
If yes, delete the Monzo app, restart your device, download & install the Monzo app and try again.
The Monzo app can be sensitive with restored data on new iOS devices.