How can TT think I had a HDHP in 2018 when I didn't? I have a form 5498-SA for a HSA, but it was not a HDSA.

[BMcCalpin]

HDHP refers to a High Deductible Health Plan. This is a health insurance policy that happens to have high deductibles.

HSA refers to Health Savings Account. The is a financial account where you put money that can be used to pay the high deductible with tax-advantaged dollars.

You cannot contribute to an HSA without having HDHP coverage. Therefore, if you had an HSA in 2018, it is highly likely that you had HDHP coverage in 2018. If there is a code W in box 12 on your W-2, then your employer certainly thinks you have HDHP coverage.