Payment assistant/ Payment voucher problem

The ‹ Payment voucher › is not pulling up the list of Customer Invoices that need to be paid. What can be wrong?

If I click ‹ Confirm payment › I get

Please help.

I have made some progress: I could pull up the lines to pay by Ventilating the customer invoice. The question is, can I use Payment Assistant/Payment Voucher to register advance payments received before the customer Invoice is even created? What is the correct procedure to do that?