How do I obtain my daughter's 2018 AGI? I prepared her return using my Turbo Tax account last year.

[VolvoGirl]

You have to log in using the exact username and password.  But Online accounts are only good for 1 return so if you did another return (like yours)  after hers in the same account then the second one overwrote her return.

How to find the AGI.
https://ttlc.intuit.com/community/agi/help/how-do-i-find-last-year-s-agi/01/25947

So hopefully you have another account.   First LOG OUT of whatever TurboTax account you're logged into right now. Then use this TurboTax account recovery website to get a list of user ID's for an email address. Run the tool against any email addresses you may have used
https://myturbotax.intuit.com/account-recovery/

If you used the Desktop CD/Download program then the only copy is on your computer and not saved or stored online.