In TaxSlayer ProWeb, if the taxpayer is making a partial payment of their federal balance due by direct debit, the program doesn't provide a way to print Form 1040-V, the federal payment voucher, for the remainder.
If you'd like to print a payment voucher for the remainder, you can fill in and print or save a PDF copy at the IRS website here. You can also direct them to irs.gov/payments where they can pay the remainder.