What is it you are trying to do?
Are you trying to pay a freelancer, as a client yourself?
Or are you trying to refund a client you have worked for?
As a client you need to have an automatically chargeable payment method on file, which is either a proper credit card, or a PayPal account if that is available in your country.
As a freelancer who wants to refund a client and who does not have enough balance in their account, you should contact support, chances are your only option will be sending money to Upwork via PayPal., for this to be applied to your Upwork account, and then for you to refund your client.