Do NOT add 2018 income to your 2019 tax 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)
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 by "last year's taxes" you are referring to a 2018 tax return. You can not use the online version of TurboTax to file a prior year tax return. You have to purchase the CD/Desktop version and physically install it on your MAC or Windows computer. Then use the program you installed to complete your 2018 tax return. Additionally, a prior year tax return can not be e-filed. The IRS says so. You have to print, sign and mail it to the IRS. You should also complete and mail your 2018 tax return before you even start your 2019 tax return.
You can purchase the CD/Download version of TurboTax 2018 at https://turbotax.intuit.com/personal-taxes/past-years-products/