You have to add a valid payment method, either a proper credit card (not a prepaid etc card) or a verified PayPal account (not available as a payment method in all countries) to your account and that is what will be charged by Upwork for your freelancer's work.
Please update the billing methods you used before to pay your freelancers by going to your client profile > Settings > Billing Methods. Please let us know if you need further assistance.
re: "I need to fund my account balance to pay my freelancer. Is it possible?"
It's a cool idea.
But the short answer is: No, you can't do pre-fund an account balance to pay for hourly work.
But that IS sort of the expected way to pay for fixed-price work.
You fund an escrow account that is largely under your control. And you can get the money back if the freelancer attached to the contract never does anything and doesn't submit the work.
With hourly contracts, Upwork is not likely to allow you to prefund a payment account. You do not know ahead of time how many hours a freelance will work, and Upwork wants to make sure they can cover the payment. So they want a more open-ended line of payment from a credit card or an appropriate PayPal account.