I can not link a credit note to an invoice as I get an error "delete extra lines". My ageing is therefore not correct.

This isn't the kind of experience we want you to have while linking a credit note to an invoice in QuickBooks Online (QBO), @ernest3. With this, I've come to guide you on the actions you need to take care of this matter.

You can use a credit note to immediately reduce your customer's current open balance and use it as a payment for an invoice. This sales form error can sometimes be caused by the browser's overtime collection of data from frequent page visits. To isolate this matter, you can perform some troubleshooting steps.

You can start by pulling up your QuickBooks Online (QBO) account using a private browser (incognito). Here's how:

• Press Ctrl + Shift + N (Google Chrome)
• Ctrl + Shift + P (Firefox)
• Control + Option + P (Safari)

Once signed in, go to the Receive Payment page and link the credit note to your customer invoice. For the detailed steps, follow these steps:

1. Go to the + New button.
2. Select Receive payment.
3. In the Customer dropdown, select the customer.
4. In the Outstanding Transactions section, select the open invoice you want to apply the credit note to.
5. In the Credits section, select the credit notes you want to apply.
6. For the open invoice in the Payment column, enter how much of the credit you want to apply.
7. Leave the Payment method, Reference no, Deposit to, and Amount received fields blank.
8. Fill out the rest of the form, including the Payment date.
9. When you're done, select Save and close. 

If you've successfully linked the credit note to the invoice, return to your default browser and clear its cache. This will refresh the system and remove older data that causes viewing and performances issue. However, if this matter continues, I'd recommend using other supported browsers.

Once you're done, I'd recommend pulling up the Invoices and Received Payments report. This way, you can effectively monitor all the payments you've received and the invoices that go along with them and keep track of your sales transactions.

Also, feel free to visit our Sales and customers page for more insights about managing your company's income and customers.

In case you have other concerns about reducing customer balance using a credit note in QBO, just let me know in the comments below. I'll be glad to help. Take care always.