If you are referring to enter your 2018 AGI on the 2019 tax return, that is a requirement from the IRS and has nothing to do with who you use to assist in filing your taxes. All tax preparation software and all tax return preparers need this 2018 AGI to e-file your return, it is your electronic signature. Otherwise, if you do not provide the 2018 AGI then the tax return can only be printed and mailed.
If you did not file a 2018 tax return then enter a 0 (zero) for the 2018 AGI.
If you did file a 2018 tax return the 2018 AGI is on your Form 1040 Line 7
https://ttlc.intuit.com/community/agi/help/how-do-i-find-last-year-s-agi/01/25947
To access your current or prior year tax returns sign onto the TurboTax website with the userID you used to create the account - https://myturbotax.intuit.com/
Scroll down to the bottom of the screen and on the section Your tax returns & documents click on Show. Click on the Year and Click on View adjusted gross income (AGI)
Or go to this IRS website for free federal tax return transcripts - https://www.irs.gov/individuals/get-transcript
If your tax return is rejected then you will have to print and mail the tax return.
See this TurboTax support FAQ for printing a tax return for mailing - https://ttlc.intuit.com/community/printing/help/how-do-i-print-and-mail-my-return-in-turbotax-online...
