You cannot enter a 2016 form W-2 on your 2018 tax return.
If you have filed a 2016 tax return but did not report that W-2, you'll need to amend your 2016 tax return to add it.
Please see the
following TurboTax FAQ for
instructions on how to amend your tax return in TurboTax.
https://ttlc.intuit.com/replies/3288565
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