Can I file my 2018 W2 this year if I didn't file last year?

You cannot put it on the 2019 online software.  Online is ONLY for 2019 income--no other year.

Online preparation and e-filing for 2018 is permanently closed.

To file a return for a prior tax year

If you need to prepare a return for 2016, 2017, or 2018 you can purchase and download desktop software to do it, then print, sign,  and mail the return(s)

https://turbotax.intuit.com/personal-taxes/past-years-products/

Remember to prepare your state return as well—if you live in a state that has a state income tax.

When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s.  Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.

Note:  The desktop software you need to prepare the prior year return must be installed/downloaded to a full PC or Mac.  It cannot be used on a mobile device.

If you are getting a refund, there is not a penalty for filing past the deadline.  If you owe taxes, the interest/penalties will be calculated by the IRS based on how much you owe and when they receive your return and payment. The IRS will bill you for this; it will not be calculated by TurboTax.

https://ttlc.intuit.com/questions/1900990-how-do-i-file-a-prior-year-tax-return

No, you cannot file a W-2 from 2018 on your 2019 return.

If you filed a return in 2018 and omitted that W-2, you'll need to amend your 2018 tax return to add that W-2.

Please follow the instructions in this TurboTax Help topic to amend your 2018 tax return.

If you did not file a 2018 tax return, you can file it now to report that W-2.

Please follow the instructions in this TurboTax Help topic to prepare your 2018 tax return.