Depends on when it was installed.
1) IF installed in 2018, then you have to amend your 2018 tax filing....and that 2018 software hasn't been updated yet to include the ability to get that credit (even though can enter that installation, it is not used and no credit is applied ...yet.... ). Since the current 2019 software season is taking precedent on a massive scale, I suspect TTX won't get around to changing it for a few months. Check back in say, August, you have 3 years to amend so plenty of time.
2) IF installed in 2019, then once you start the actual AMEND process, it's on the "Deductions&Credits" page, at the "Home Energy Credits" menu start point. Once you start in there, the air conditioners are part of the line labelled " Energy-Efficient Building Property" (terminology which many people miss ).
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*
