I own Game Pass, I loaded up Halo Reach and it says I don’t have it installed, but in the menus it states that it is installed. So I attempted to purchase it to see if that would fix the issue, and it did not fix it.
Not sure if this will help or not but try referring to this link. - It’s a link to another halo waypoint forum in which someone else was having problems launching reach as well and seemed to have found a fix.
You already have a topic on this, no need for another