This is a glitch in TT.  Since TN does not have an income tax, just leave boxes 15-20 blank at the W-2 screen. This assumes box 17 on the paper W-2 is 0 or blank.  TT does not use those numbers for anything. 

@Hal_Al 
(From TN) Thank you!!
I’ve been searching online for why this exact thing is happening to me, and even called the turbo tax support line to no avail. I’m assuming this is a new/recent glitch, because the past 7 years I’ve been able to input that information.

Yes. The glitch is new this year. They are aware of it.  I suspect it's low priority with them