I have education expenses for 2016 and 2017 but I didn't file taxes for those years, so how can I add an American Opportunity credit for 2017 to my 2018 return?

No, you cannot claim a credit for 2017 on this year's return. Since you never filed you can file prior year returns for 2016 and 2017 until April 15th, 2020. 

Specifically, you have up to three years past the original tax deadline to claim a refund you're entitled to. So, you can file your 2016 tax return any time before April 18, 2020 (October 17, 2020, if you have an extension).

Here's how:

https://ttlc.intuit.com/replies/3301770