Pre-requisites for all the events (which also includes Promotion event, yes), are listed in the "Events Log" tab in the "Journal," (which is the J button at top).
If you can't trigger that rank up event, it means most likely you haven't fulfilled one of the requirements needed to trigger that event.
One other possibility is that the event is bugged, but as far as I know (at least in the PC version), with v0.08.0.3, all bugs have been quashed.
But if you're playing it on android, well..... that's a different story.
If that's the case, you'll likely need to ask the person who ported the game to android (since dev didn't do the android port).