You manually enter the AGI when you go through the File tab.

How to find the AGI https://ttlc.intuit.com/replies/3301618 

If I have already filed my 2018 tax return and it still says that I need to upload my 2017 AGI in order to receive my return where do I go to do so? Do I need to complete an amended return at this point, I am very confused by this entire situation and TurboTax has no instructions on how to do so

Was your return Accepted?  If you didn't have the AGI it should have rejected.
You can check your efile status here     <a rel="nofollow" target="_blank" href="https://turbotax.intuit.com/tax-tools/efile-status-lookup/">https://turbotax.intuit.com/tax-tools/ef...>

There is nothing that you need to “upload”. All you have to do is type in a number.

Where? TurboTax only allows me to go back in and amend the Tax documents but does not tell me where to input this number