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?
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