I have all my W-2s for 2018 and I'm interested in filing the taxes for 2018. Is this possible to file them now? I have already filed my taxes for 2019. Thanks

The current  online software is ONLY for 2019 returns --which is what everyone is now preparing for the 2019 tax year that just ended.  You cannot use online for any other year.   And you have until April 15, 2020 to file your 2019 return. 

Prior year returns can only be done using desktop software and the returns must be mailed in.

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.

Or If you have a simple return You can fill out the blank forms for free.
Here is the new IRS 2018 Form 1040
https://www.irs.gov/pub/irs-pdf/f1040.pdf
And instructions https://www.irs.gov/pub/irs-pdf/i1040gi.pdf
2018 Tax and EIC Tables https://www.irs.gov/pub/irs-pdf/i1040tt.pdf

IRS 2018 Schedules 1 thru 6
https://www.irs.gov/pub/irs-pdf/f1040s1.pdf
https://www.irs.gov/pub/irs-pdf/f1040s2.pdf
https://www.irs.gov/pub/irs-pdf/f1040s3.pdf
https://www.irs.gov/pub/irs-pdf/f1040s4.pdf
https://www.irs.gov/pub/irs-pdf/f1040s5.pdf
https://www.irs.gov/pub/irs-pdf/f1040s6.pdf

Don’t forget your state. And you will have to print and mail your returns.  So be sure to attach copies of your W2s and any 1099s that have withholding on them.  You have to mail federal and state in separate envelopes because they go to different places.  Get a tracking number from the post office when you mail them for proof of filing.