Thanks for posting here, @delta-gama. I'm here to ensure you'll be able to apply a credit memo to the invoice in QuickBooks.

First off, let me discuss information about the process.

In QuickBooks Desktop for Mac, the steps in creating a credit memo or credit note and linking it to the existing invoice of the customer were already provided by my colleagues above. For reference, feel free to check this article: Create and apply credit memos in QuickBooks for Mac.

If you're having trouble performing the task, I recommend reaching out to our Technical Support team to assist you in creating a credit note. Once connected, you can also request a screen-sharing session so they can further view and check your set-up.

Additionally, here's a helpful resource that you can access to look for guides whenever you encounter navigating your QuickBooks account: QuickBooks Help page.

Feel free to post here again if you have any other concerns in creating a credit memo in QuickBooks. I got your back and I'm ready to lend a hand. Have a good one.