It won’t accept answer TN for box 15 and that is what’s is on my w2

Since TN does not have state income tax, are boxes 16 and 17 blank? You do not need to enter Box 15.

Alaska, Florida, Nevada, South Dakota, Texas, Washington, and Wyoming don't have income tax either, but they're all listed in the dropdown list.  I'm thinking if there's something in Box 15, we should add it in TurboTax the way I've done for the past 30+ years.  It seems like it's just an oversight this year. (I'm using TurboTax Home and Business).

The state code in Box 15 is there to reference what state the withholdings in Box 17 belong to.  If  Box 17 is blank (it should be in a state with no income tax) then Box 15 is not needed.   Some employers automatically include the state code, but if there is no entry in Box 17, then Box 15 is not required.    

It probably should be in the list though, because as you stated, all of the other 49 (even the other non-income tax states) states are listed.   Thank you; we will get the list updated!