Hey there, @sales321.

If the invoices under To Link Invoice on the Credit note are not appearing when you try to apply them, the most likely reasons are:

• The customer names on the invoice do not match exactly.
• If multi-currency is enabled, both the invoice and credit note should be in the same currency.
• The credit note must still have a balance available to use. If it was already applied or closed, it won’t appear as an option. 

To verify this, go to the customer’s profile under the Sales menu, locate the credit note, and review its transaction history for any discrepancies. Here's a visual reference:               

Once you’ve verified the details, you can link the credit note to the invoice by following these steps:

1. Click the + New button on the left-hand menu.
2. Select Receive Payment.
3. From the Customer dropdown, choose the correct customer. Ensure the customer has both the invoice and the credit note associated with them.
4. Under the Outstanding Transactions section, check the box next to the invoice you want to apply the credit to.
5. In the Credits section, check the box next to the credit note you want to use. QuickBooks will display the amount applied, reducing the balance on the invoice.
6. Leave the Payment method, Reference no, Deposit to, and Amount received fields blank.
7. Fill out the rest of the form, including the Payment date.
8. Double-check that the total is accurate after applying the credit note.
9. Click Save and close to finalize the transaction.                                                  

You can refer to this article for more information: Create and apply credit notes or delayed credits in QuickBooks Online.

If you have additional questions or need assistance, don’t hesitate to reach out. The Community space is available 24/7.