What is actually happening is this:
There is a question that every taxpayer (with an HSA or not) should answer: "What type of High Deductible Health Plan did [name] have on December 1, 2016?" The reason for this question is that IF you had an HSA in 2016 and IF you contributed under an obscure rule called the "last-month" rule to use the full annual HSA contribution limit even though you were not under HDHP for all of 2016, then you are required to have stayed under HDHP for all of 2017.
Really.
But if you didn't have an HSA at all, you didn't go through the HSA interview. If you didn't go through the HSA interview, you never showed that you had HDHP coverage in 2017 (of course not).
But when you answered "Family" or "Self" to the question "What type of High Deductible Health Plan did [name] have on December 1, 2016?", you told TurboTax that you had HDHP coverage in 2016 - although this was often by mistake. You apparently had coverage in 2016 but no coverage in 2017, so TurboTax thinks that you ran afoul of the last-month rule. But if you had no HSA and no HDHP coverage in 2016, then you should have answered "None", because the last-month rule didn't apply to you.
Yes, it is obscure. The problem is often rooted in the fact that when TurboTax asks about HDHP coverage, many taxpayers who don't have HDHP coverage don't realize that this question doesn't even apply to them, so they should answer "None". Yes, I am sure that next year, it will be clearer.