Need help understanding the Health Marketplace Tax Credit

I am trying to fill out my son's return for him using TurboTax.  He works various part-time jobs, none of which offer health insurance.  He received a form 1095A for his marketplace plan.  Column A showed his monthly premium, but Column B and C were blank, which is how it's been in prior years.  I went to the IRS credit calculation tool and found the SLSCP premium amount and entered that into Column B.  He did not receive any advance tax credit, so I left Column C blank (and entered $0 when it complained about it being blank).  In the past 3 years, TurboTax computed his tax credit just fine, but this year it's computing $0, and instead putting all his premiums into itemized deductions (he takes the standard deduction)

 

The income eligibility rules say that income must be at least 100 percent but no more than 400 percent of the federal poverty line for their family size (he's single).  The FPL for 2025, for a single person, appears to be $15,650.  His total income is ~$18,000, so it seems to me like he should qualify for the tax credit.

 

Why is TurboTax not calculating a credit?