You will need to elect a billing method first before the system starts charging your Upwork balance for your Freelancer Plus Membership. If you have a sufficient available balance from Upwork earnings, we will use that to pay for your membership. If your balance isn't sufficient, we'll charge your chosen billing method(Visa/MasterCard/Discover/American Express/PayPal). If your billing method fails for any reason, you'll be immediately downgraded to a Basic plan. Please fix the issue with your billing method and then you can re-enroll in a Plus plan.
You may read more about it here.