If you are working on your 2017 tax return, you cannot claim premiums you paid in 2016.
In order to claim premiums paid in 2016, you will need to amend your 2016 tax return to add them. To amend your 2016 tax return, please follow these instructions:
https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed
Before you amend your 2016 tax return, please know if your premiums are paid for with after-tax dollars, then you may deduct as a medical expense on Schedule A. The total medical expenses will need to exceed 7.5 % of your AGI to receive a benefit. If the premiums are paid with pre-tax dollars, they are not deductible.