If you choose "App engagement" as your campaign goal, you can optimize for clicks and impressions. If you also want to optimize for installations, choose "App installs" as your goal instead. Install optimization will be available once there are 5 or more downloads on the same tracking link within 48 hours. This applies to CPM (cost-per-mille impressions) and CPC (cost-per-click) campaigns. That's why it's important to configure "App Install" event notifications through the myTracker mobile analytics service or another tracker.