This answer has two parts, depending on your situation. Please read both.
If you have an HSA and HDHP coverage:
The section that Line 18 is in is on form 8889 in the section referring to “Failure to maintain HDHP coverage”. TurboTax is asking for information from your 8889 on your 2017 return.
You should have triggered this when
1. you indicated that you were in the HSA on December 1, 2017 (yes, 2017), and
2. you indicated that you were not covered by the HDHP in every month of 2018.
If you are in this situation, you must complete the questions on the next screen – which asks you for some information from your 2017 HSA in order to be able to fill in Line 18.
So, if you are in this situation, then go find your 2017 return and enter the information from the 8889.
If you don't have an HSA or even know what HDHP coverage is:
HDHP refers to a High Deductible Health Plan, a type of health insurance policy with high deductibles compared to other policies. An HSA (Health Savings Account) is an account that lets you pay the high deductible of the HDHP with pre-tax dollars.
If you don't have an HSA or HDHP coverage, then you probably answered a question that made TurboTax think that you had an HSA. You likely did this when you answered "What type of High Deductible Health Plan did [name] have on December 1, 2017?" with "Family" or "Self" when you should have answered "None" because you don't have HDHP coverage.
In this case, I suggest you delete your HSA entries and start over.
The safest - although brute force - way to fix this is to do the following in this order:
1. make a copy of your W-2(s) (if you don't have the paper copies)
2. delete your W-2(s) (use the garbage can icon next to the W-2(s) on the Income screen
*** Desktop***
3. go to View (at the top), choose Forms, and select the desired form. Note the Delete Form button at the bottom of the screen.
*** Online ***
3. go to Tax Tools (on the left), and navigate to Tools->Delete a form
4. delete form(s) 1099-SA (if one), 8889-T, and 8889-S (if one)
5. go back and re-add your W-2(s), preferably adding them manually
6. go back and and continue your return.
***NOTE*** If you did have an HSA in 2017 but thisngs have gotten confused, then do the procedure above to delete all your HSA information and redo the HSA interview (Federal->Deductions & Credits->Medical->HSA MSA 1099-SA or Search for hdhp (lower case) and jump to hdhp).
