I hear your sentiments, Mandy, and understand the frustration of the existing issue when applying a credit memo to an invoice. Don't worry, I'm here to ensure you can achieve the task in QuickBooks Online(QBO).

When applying a credit note as a payment against an open invoice, make sure to remove any existing line items. Once confirmed but unable to link it, we can do some basic troubleshooting steps.

Have you performed the process shared by my colleague, MJoy_D, above? If not, I suggest doing so to determine if this has something to do with your browser. We can start by opening your QBO using a private (incognito) window. You can utilize these keyboard shortcuts to save time.

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

Once signed in, apply the credit memo to an invoice again. If it works, return to your regular browsing history and clear the cache. Otherwise, you can try other supported browsers to help narrow down the cause of this behavior.

If the issue remains even after following these steps, I recommend reaching out to our Phone Support team since they have all the tools to further investigate the issue. 

Moreover, you might want to consider applying your credit note to an invoice using the Recieve Payment feature. All you have to do is enter the product as a line item within the credit note and then apply it to an invoice. I'll walk you through the process:

1. Click the New or Plus sign (+) button, and choose Receive payment.
2. Select the customer.
3. Tick the Invoice under Outstanding Transactions, and the Credit Note under Credits.
   
   
    
4. Hit Save and close once done.
   
    

You'll want to visit this article as your future reference if you want to return your customer's money: Record a customer refund in QuickBooks Online.

If you have more questions about the process, don't hesitate to reach out to us again, Mandy. We're available 24/7 to help you.