If you're attempting to activate auto top-up and encounter a message stating, "We're unable to set up auto top-up as you don't have a default payment method," it's likely because no payment method has been designated as default on your account.
To establish a default payment method:
Click the grey icon located at the top right corner of the screen.
Choose 'Subscription' from the menu.
Select 'Set up BACS Direct Debit' and input your payment details when prompted.