A 2019 tax return will not have the 2018 AGI entered on the return. The 2018 AGI is only needed to e-file a 2019 tax return.
Are you trying to file a 2019 tax return? If so, you cannot use the TurboTax online editions as those editions are only for tax year 2020.
To complete and file a 2019 tax return using TurboTax you would need to purchase, download and install on a personal computer one of the 2019 desktop editions from this website - https://turbotax.intuit.com/personal-taxes/past-years-products/
A 2019 tax return can only be printed and mailed, it cannot be e-filed.
Your 2018 AGI is nowhere on your 2019 return. Your 2018 AGI was only required to identify you when *e-filing* a 2019 return which can no longer be e-filed at all so the 2018 AGI is not needed for anything now. 2019 can only be paper filed and mailed.
Why? What are you trying to do? Your 2018 AGI is not on your 2019 tax return. Your 2018 AGI is on your 2018 tax return. You needed to enter your 2018 AGI in order to e-file a 2019 return----but it was not "entered" on the actual Form 1040. Entering that AGI just confirmed your identity to the IRS to allow you to e-file. it is now too late to e-file a 2019 return, so if you are trying to find it to e-file, you do not need it. A 2019 return can only be filed by mail now. it must be printed, signed in ink, and mailed to the IRS.