Turbotax did not include a credit in my 2017 NY state return for non-resident withholding taxes I paid to Illinois (even though filed IL through Turbotax). How to fix?

You do not get to deduct all the withholding ... only the taxes paid to IL on the 2017 return. If you had completed the non resident state first then the program would have moved that amount automatically.  If you didn't then you will need to amend the state return.

Tks.  The IL taxes were paid before filing and I filed IL and NY at the same time.  In any case, Turbotax won't let me amend NY to reflect the IL taxes paid (the info is already in the program but not on my 2017 NY state return for some reason).  2018 reflected it correctly, which is how I noticed the error in 2017.

Interesting ... when you click on the Amend choice and start the amendment process will the program not let you click on the state tab to amend the NY return ?