The IRS requires that your Adjusted Gross Income (AGI) from your tax return from the previous year be included with any tax return e-filed this year.
You can find your 2018 AGI on Line 7 of your Form 1040.
If you had filed with TurboTax last year, the AGI would have been automatically added to the return. If you did not file with TurboTax last year (or if you filed in a different TurboTax account), you will need to enter the AGI from last year manually. The AGI is added near the end of the filing process, just before filing the return.
To find the AGI from a different TurboTax account, sign in to that account, then scroll down on the opening page until you get to Your Tax Returns and Documents. Chose 2018 and download your tax return.
If your previous year tax returns are not available, you can use the IRS Get Transcript tool. Try the online option first. If you are unable to obtain the transcript online, you can have one sent to the address the IRS has on record for you. You can find IRS Get Transcript here.