Hello there, SummitComp:

Ideally, you can apply for the credit from the Invoices tab, then Receive Payment option. Can you send a screenshot of the steps that you've tried? This way, we'll be able to check if the correct process has been followed when applying a credit to an invoice. 

Anyway, if there's anything else that you need help with, let us know so we can get back to you as soon as possible. Have a great day ahead!