You cannot file a 2018 W-2 with your 2019 tax return. You would have to & should amend your 2018 return to add the W-2. If you owe on the 2018 W-2, penalties & interest will accumulate until the taxes are paid in full. Amended returns must be mailed & they can take 16 weeks or more to process.
If you filed with TurboTax for your 2018, you can use TurboTax's amendment software.
https://turbotax.intuit.com/personal-taxes/cd-download/amend/ty18
How do I amend my 2018 TurboTax Online return?
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Three weeks after mailing your amended return, you can start tracking it at the IRS Where's My Amended Return? tracking tool. Please allow at least 16 weeks for the IRS to process your amended return.
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