PayPal Won't Let Me Pay Without A Credit Card

Why does this happen?

This happens when you attempt to use Paypal’s subscribe feature without having any form of bank, credit card or debit card added to your Paypal account.

How can I pay without a bank, credit card or debit card being added to my account?

In the invoice, simply click Paypal Checkout instead of Paypal Subscribe. As a result of this you will have to pay invoices manually, invoices will be generated prior to the due date, you can pay invoices in the Invoices tab in the client area.