DId you use TurboTax to complete and file your 2016 and 2017 returns? If so, did you use the online version both years? Or did you use the CD/Desktop version both years?
But basically, the answer to your question is yes. You are required by law to depreciate rental property. If you don't, then when you sell the property you have to deduct from your cost basis the depreciation you "should" have taken, and that means you are double-taxed on that money as your penalty for not taking depreciation.
If you used the TTX program both years, then you will need to amend the 2016 return first, so that you will have the correct amounts of prior depreciation to enter in the 2017 program when you amend it.
Do you know "for a fact" that you did not take depreciation on the property in 2016 and 17? If you used TTX those years and used the program the way it is designed and intended to be used, then it would be rather difficult to not have taken the depreciation. If anything, you can look at the IRS Form 4562 for that property for each year, to confirm depreciation one way or the other.
