You are able to file your 2018 return with TurboTax. Click the following link to select which 2018 version would best fit your needs: https://turbotax.intuit.com/personal-taxes/past-years-products/
You should file 2018 first, then work on 2019. There might be information from your 2018 return that needs to be entered on your 2019 return.
Click here to chose which version of 2019 would be fits your needs: https://turbotax.intuit.com/personal-taxes/online/
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You cannot combine income from two different tax years on the same tax return. If you do that it will be a mess that will take months for you to straighten out. That is not TurboTax making life hard for you--that is TT telling you the way tax laws work. Each year has to be filed separately using the forms and software for the specific year. The online program can only be used for 2019--no other year.
To prepare and file a 2018 return:
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)
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.