A category assigned to an applied credit must match the sale's category

If you assigned a category to a credit, it must match the category assigned to the transaction in the sale to which the credit is applied. If the categories don't match or if either the credit or the sale has no category assigned to it, the sale will not appear in the list of sales.

Use the Open Invoices view or Orders view of the Sales Register to find the invoice or order to which you will apply the credit. Then assign a matching category to a transaction in the sale.