Thanks for bringing this to our attention, lawrenanne1. Allow me to explain how the Assess Finance Charges feature works in QuickBooks Desktop (QBDT).
In QuickBooks Desktop, the Assess Finance Charges feature is applicable only for invoices that remain open or unpaid. Given that you have already received payment, the relevant customer will not appear when you attempt to assess the finance charge.
To assess finance charge for your customer, you'll need to unlink the payment to make the invoice open. Then, you can proceed to Assess Finance Charges for that customer and link the payment again once you're done.
Please note that the Assess Finance Charges will automatically create an invoice for the finance charge that you'll need to send to your customer so they can pay.
Here's how to do it:
1. Go to the Customers section then Customer Center, and select your customer.

2. Click on the Payment transaction and uncheck the box.

3. Once you’ve made the change, click Save & Close, then Yes to confirm the changes.

4. Hover your cursor over your customer’s name and right-click.
5. Select Assess Finance Charges.

6. Set the Assessment Date to a date after the due date of your invoice.
7. Check the box to assess your customer for finance charges.
8. Hit Assess Charges and then Yes to continue.

9. Return to the Payment transaction, check the invoice box, and click Save & Close.


10. Your customer will now have an open invoice that was automatically created after the Assess Finance Charges.
You can refer to this article for detailed information about Assess finance charges: Assess finance charges.
Additionally, you can run and customize sales reports in QBDT to gain an overview of your customers' finance charges.
Keep us posted if you have other concerns about finance charges or any QuickBooks-related tasks. We're always here to lend a helping hand.