You cannot contribute to an HSA for any months that you are covered by Medicare. Most people sign up for Medicare at age 65, so if you did so you are ineligible to contribute for those months. If your Medicare coverage began January 1, 2018 or earlier, you cannot contribute to an HSA for 2018 and beyond and your only option is to claim the 2017 expense as a deductible medical expense if doing so will increase your itemized deductions for 2017 high enough.
Note that in TurboTax, when asked for your medical expenses under Deductions & Credits you must enter all of the expenses, including those paid using your HSA. TurboTax automatically subtracts from that the amount of HSA-paid expenses reported when determining your medical-expense deduction.
