Make sure you follow all the steps here https://developers.facebook.com/docs/ads-for-apps/mobile-app-ads
Especially the section "Add the Facebook SDK". In it, it tells you where to call either
[FBAppEvents activateApp]; // for iOS
or
AppEventsLogger.activateApp(...); // for Android
Which tells Facebook that the app was successfully installed. Otherwise, Facebook would not know that the app was installed.