You didn't find QGO in the unknown sources section, or you don't see the unknown sources section ?
It this is the first statement, I assume you previously had a QGO version installed in your headset ? And you uninstalled it. Am I right ?
Sometimes when you uninstall an app from meta menu, the app is not really removes by the Meta Quest system (don't ask me why)
So you need to perform a clean uninstallation (with sidequest PC app) => see the FAQ : https://anagan79.itch.io/quest-games-optimizer/devlog/538649/questions-answers
Then restart the Quest then install QGO again
