2018 can only be prepared using the desktop software which must be purchased. it can not be e-filed. use a mailing method where you get proof of delivery. follow TT filing instructions
https://turbotax.intuit.com/personal-taxes/past-years-products/
Whatever you do, do NOT file a 2018 return if you used the current online program. Online is ONLY for 2019. You cannot edit or change the year. If you will need to prepare a 2019 return then you can delete the 2018 income you entered and use the account for your 2019 return instead. Otherwise, abandon the return. It will be all wrong.
Each tax year has its own software and tax forms that you must file the income on. You need 2018 software for 2018 income.
Online preparation and e-filing for 2016, 2017 and 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/
You may also want to explore purchasing the software from various retailers such as Amazon, Costco, Best Buy, Walmart, Sam’s, etc.
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.