In order to e-file your return through TurboTax, you will need your AGI from last year. Your prior year AGI is used to confirm your identity with the IRS when you e-file.
If you do not have a copy of your 2017 Tax Return, you can order a free transcript online from the IRS by clicking here: Get My Transcript. Your IRS transcript will include your AGI from last year.
For more information, you can check out the TurboTax FAQ link for more details on Finding Your 2017 AGI.