Unless he is claiming a dependent or he is in Alaska, I can't figure out why it isn't calculating a credit.
Have you finished the rest of his tax return and gone through the error-check/review, in case that comes up with something? And double-checked that no credit is showing up on line 9 of Schedule 3 (which flows to line 31 of Form 1040)?
Unfortunately, I'm out of ideas, so you might need to call Customer Support, unless anybody else comes along with an idea.
