My wife lives in KY but works in TN, it is saying that we need to file TN state taxes, but never have in the past. Why is it saying that?

Not sure.

I recreated your issue. I don't get this message. When I report TN income as a KY resident, the program prompts me to start a TN tax return, but then it tells me I don't need to file it. 

Regardless, Tennessee doesn't have state income tax on wages, so you don't need to file a Tennessee state tax return to report her earnings.

As you know, you do need to report this out of state income on her Kentucky state tax return. 

I just added w2 like I always do, it should populate on KY return, correct?
anyway to check this before paying and filing?

Correct, look at the KY Summary in Tax Tools at the bottom of the black column on the left