A category assigned to an applied debit must match the purchase's category

If you assigned a category to a debit, it must match the category of the purchase to which you apply that debit. If the categories don't match or if either has no category, the purchase will not appear in the list of purchases. Use the Open Bills view or Orders view of the Purchase Register to find the bill or order to which you will apply the debit. Assign a category to one of the transactions so they do match. Return to this view and the purchase will appear.