Thank you for your prompt reply. I'll take care of your query about the A/R Aging report.
As stated by @BigRedConsulting above, the option you're looking for is unavailable. What you can do is follow the steps shared by @katherinejoyceO when you create an invoice for the customer that you wish to apply the credits to.
Take a look at this article as well: Advanced troubleshooting for balance sheets out of balance. It's a detailed guide on how you can run custom reports to identify the cause of an out of balance Balance Sheet.
Direct other questions you have in the comments below. I'll get back to you as soon as I can.