The update to iOS 26 is naturally absolutely crucial for this to work, I don't know why I expected the new Activity functionality to run on iOS 18.6.. Sorry about this, a very avoidable mistake on my end, especially with that very clear reason given with the exception.

We're currently facing the same error with live challenges Invalid game activity definition. Failed to kick activity notification to GameKit. Error: Error Domain=GKErrorDomain Code=17 "(null)" For testing, the Xcode GameKit debug menu links work as expected on our side as well. The example "live" link on App Store Connect in the section Deep Linking > Start Activity Intent (https://games.apple.com/us/activity/start?a=&b=&g=<...>) doesn't work with the same error (only noticed this after the fact unfortunately). After comparing this example "live" link with the Xcode GameKit debug menu link, the difference seemed to be in the additional parameters for the debug link: the language and leaderboard parameters were not present in the "live" link example. After adding only the language parameter to the "live" link example, it started to work as intended as well. Unfortunately I don't know what to make with this info, if there is some problematic setup with the Activity data on App Store Connect that causes this, or if this can point to other issues with the overall setup. It might still be interesting if your "live" link example displayed on App Store Connect for your activity also starts working with the language parameter, but I'm not sure if it gets us closer to a solution unfortunately. Cheers!