If you haven't filed 2017 yet then enter 0 for the AGI to efile 2018.
You can only buy the download for prior years. You need a full Windows or Mac computer to install it on. Buy the download here
https://turbotax.intuit.com/personal-taxes/past-years-products/ You have to print and mail prior year returns. 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 received the return. Don’t forget state.
If you are getting a refund, there is not a penalty for filing past the deadline. You have 3 years to file for a refund. 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.
How to file a prior year
https://ttlc.intuit.com/questions/1900990-how-do-i-file-a-prior-year-tax-return Or If you have a simple return You can fill out the blank forms for free.
Here are some blank forms & instructions for 2017
1040
https://www.irs.gov/pub/irs-prior/f1040--2017.pdf Instructions
https://www.irs.gov/pub/irs-prior/i1040gi--2017.pdf 1040A
https://www.irs.gov/pub/irs-prior/f1040a--2017.pdf Instructions
https://www.irs.gov/pub/irs-prior/i1040a--2017.pdf 1040EZ
https://www.irs.gov/pub/irs-prior/f1040ez--2017.pdf Instructions
https://www.irs.gov/pub/irs-prior/i1040ez--2017.pdf 2017 Tax Tables
https://www.irs.gov/pub/irs-prior/i1040tt--2017.pdf Publication 17 Tax Guide for 2017
https://www.irs.gov/pub/irs-prior/p17--2017.pdf OR did you start 2017 Online last year? How to finish a prior year online return
https://ttlc.intuit.com/replies/4186197
