Here is a Mac How To with an image, that shows the credit is seen in the pay bills listing:

Or, I believe your bill Image will show available Credits/Discounts, a button at the bottom of the bill as you view the bill.

Resolution for 'available Credits issue: 'Vendor overpayment on Mac' available: Yes.

The available Credits issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the available Credits issue yourself by looking for a resolution described in the article. If it is a complex available Credits issue or you are unable to solve the issue, you may contact us by clicking here or by using other available Credits support options.

For more information about resolving issues related to available Credits in Quickbooks/QBO or to get support for the issue 'Vendor overpayment on Mac', click the link below: