It means you're trying to download a beta version of the app.
Beta version means it's in testing phase and there are only a limited number of people using it because it needs to be a controlled test. For example I want only 100 people to be beta testers. Then only 100 people can download it. If the 101st person tries to download, he gets a beta is full error.
On the Facebook app, scroll down and there should be an option to leave beta, like the picture below. Choose to leave beta so you can download the normal version.
