What does "won't let you" mean? What/where is the roadblock?
Log into your Online account that you used for your 2018 return, go to the Tax Home and scroll down to the section "Your Tax Returns & Documents". Expand that section and choose tax year 2018. There should be a link "View adjusted gross income (AGI)". Do you see that link, and does it produce your 2018 AGI?
There is also a link in that section that says "Download/print the PDF." Is that the link that you are unable to use?
FAQ: How do I find last year's AGI?
https://ttlc.intuit.com/community/agi/help/how-do-i-find-last-year-s-agi/00/25947
Here's an alternate way to access a prior year return:
https://ttlc.intuit.com/community/prior-year-return/help/how-do-i-access-my-prior-year-return/01/270...
