Hi I can't select any credit memo when receiving a payment. The boxes with different Credit memos are disappearing when trying to select a box.

Let's get this issue sorted out so you can apply the credit memo to your invoices as soon as possible, @thomas31.

Make sure to select an invoice first so the credit memo box will appear. See the sample screenshots below: 

For more information about the process, check out this article: Create and apply credit memos or delayed credits in QuickBooks Online.

If the same behavior happens after following the steps, try logging in to a different private window. Let's check if the problem is brought about by issues in the browser and its cache.

To launch a private window, try out these keyboard shortcuts:

• Safari: Command + Shift + N
• Mozilla Firefox: CTRL + Shift + P
• Google Chrome: CTRL + Shift + N 
• Internet Explorer:  CTRL + Shift + P

Once logged in, go back to the Received Payment page and check if the credit memo is functioning well. If it works, clear the cache of your regular browser to speeds up the web browsing experience. Using another compatible browser is a good alternative too. 

You can generate the Transaction List by Customer to track all credits you've used and the remaining balance. Simply click the Customize button, then in the Transaction Type, choose Credit Memo, and select a specific customer in the Customer drop-down arrow. For the detailed steps on how to customize, visit this article: How to Customize Reports: Basics, Sorting & Date Ranges.

I've included an article that contains steps on how to process customer refunds.  

• Record a customer refund in QuickBooks Online

I'll be right here if you need further help with this topic or need anything else in QuickBooks. Have a nice day!