In the main Uber app I see an ad at the bottom of the screen You're Invited to try Eats Pass free for 1 month, but when I tap it I see, Sorry, Eats Pass isn't available to you right now. I installed the separate Uber Eats app, and I was invited to activate the American Express 12 month Eats Pass.
The Uber Eats software also appears to be more polished than the version integrated into the main Uber app. For example, when using the Reorder feature to order from Starbucks, customized drinks need to be re-entered in the Uber app, but the Uber Eats app is able to submit the prior order including all options.
Very similar experience for me just now. I logged in to Uber Eats on my desktop, clicked the button for try Eats Pass for a month and got the same "Sorry" message. I went over to my Samsung phone and used the Uber Eats app -- all went smoothly and I've activated the 12-months free deal for Amex Plat cardholders. This is another valuable Amex Plat benefit for me, worth about $120/year because I probably would have signed up for Eats Pass even if I didn't have the card.