You will need to amend the 2016 tax return to claim the credit.
See this TurboTax support FAQ for amending a 2016 tax return - https://ttlc.intuit.com/questions/3061514-how-do-i-amend-a-2016-return-in-turbotax
No. You can file the 2017 tax return now. You have up to three years from the original due date of the tax return to file an amended return showing a tax refund. So for tax year 2016 the amended return would have to be printed and mailed on or before April 17, 2020.