Every time i try to create a credit note and add the invoice needed, it says failed to add invoice. why?

Let's run some troubleshooting steps to isolate the issue, mattround2-1.

Let's try adding an invoice on a private window. It is the best place to identify any browser-related issues. You can use these keyboard shortcuts to open one:

• Google Chrome: Ctrl + Shift + N
• Mozilla Firefox: Ctrl + Shift + P
• Microsoft Edge: Ctrl + Shift + P
• Safari: Command + Shift + N

If it works, you can go back to a regular browser and clear its cache. A piled cache can also be the reason for some unexpected error on the opened pages.

You can also switch to a different browser like  Google Chrome, Mozilla Firefox, Safari, or Microsoft Edge as alternative.

I've also added this reference as your guide in applying credit memo: Create and apply credit memos or delayed credits in QuickBooks Online.

I'm just a post-away if there's anything that I can help. Keep safe!