HDHP and Medicare. How to make turbo tax accept the input?

Are you referring to the "lapse in coverage" message that you immediately see after the question "What type of HDHP coverage did you have on December 1, 2017?"

If so, then going on Medicare is indeed a lapse in HDHP coverage.

TurboTax believes that you (or your spouse) took advantage of the "last-month" rule, which permits you to use the full annual HSA contribution limit if you had HDHP coverage on December 1, 2017 (yes, 2017), even if you did not have such coverage all year.

If you take advantage of the last-month rule to increase your annual contribution limit, then the catch is that you have to stay under HDHP coverage for all of the next year (called "the testing period"). If you indicated that you went on Medicare at some point in 2018, then there was a "lapse" in coverage, because you were not under HDHP coverage all year (Medicare does not count).

You may or may not owe a penalty at this point - it depends on what your annual HSA contribution limit would have been in 2017 without the last-month rule and how much you actually contributed in 2017. Just answer the questions, and TurboTax will figure out if you owe a penalty.