If you have problems starting the EA laucher (a blank EA launcher colored screen and crashing back to the desktop / your library) this is your fix:
https://steamdeckhq.com/tips-and-guides/fixing-ea-play-blank-screen-for-ea-games-on-steam/
If the game itself doesn’t start, I don’t know how to help, sorry.
Edit: Let the program do its thing before you do anything else. Some steps require some time to be finished.
Elden Ring with the seamless COOP mod has been very fun for my two friends and me. The experience isn’t 100% perfect (some bugs) but exploring the Lands Between with buddies and fighting the bosses together has easily been in my top 3 COOP experiences up until now.