Categories assigned to credits must match the categories for the invoices or orders to which the credits are applied

If you assigned a category to a credit, it must match a category assigned to a 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, 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.