You must always have at least one bank account linked to your Card. If you want to unlink your existing account, you first need to link a new one.
To do this, log in on the mobile app and select your Account Icon in the top left, and then “Linked accounts”. Then select the new bank you want to link. Once your new account is linked you can remove the old one.
If you already have two linked accounts, you can press “Unlink” to remove one of them.