Not sure where you are seeing anything that says you are supposed to put income from a past year on your 2020 tax return. Do NOT put a W-2 from some other year on your 2020 return, Only 2020 income goes on a 2020 tax return.
If you did not file a 2018 tax return using your 2018 W-2 then you can prepare a 2018 return but it has to be on a 2018 Form 1040 and you have to use 2018 software. If you had no income in 2019 then you have nothing to file on a 2019 tax return.
What is making you think you need to put 2018 on your 2020 return?
Turbo tax kept asking for this, over and over again, they also asked me to itemize what I had bought online or out of state with a special district! tax percentage, and I've never been asked this before. I had to guesstimate. As my friend said, it appears they gave me a wrong file this year, and I don't know how it will work out this time. Thank you for your help!!