No need for you to put in a credit/debit card in the "Billing Method". Clients are meant to input their preferred billing method i.e. credit/debit card so that they can pay their hired freelancers.
Since you're a freelancer, you need to setup your payment method. Please go to your Settings. Under "User Settings" on the left-hand-side of the screen, click on Get Paid, and setup your preferred payment method. You may check out this article to know more about setting up your preferred payment method.