You had to use the 2018 Business program installed on your computer to do the Federal 1120S. You start with that return. Where is Turbo Tax asking for 2019 info? You will need to buy the state program from inside the 2018 Business program.
So you first have to run the 2018 program and open your 1120s return in it. Do you still have the Business program installed?
I filed my 2018 Federal Form through snail mail. Wisconsin is now requiring companies to file electronically and so I am trying to file my 2018 Wisconsin Form 5S electronically, but when I purchased Turbo Tax, it was for 2019.
Don't think you can efile a business return for 2018 anymore. Maybe from a local CPA. Why are you trying to buy the program? You should have it from doing the Federal. Did you use Turbo Tax to do the federal 1120S? If you didn't use Turbo Tax then you can't use Turbo Tax to do the state.
You can buy 2018 from here
https://turbotax.intuit.com/personal-taxes/past-years-products/