Are you seeing this message when trying to install an iOS Enterprise app in iOS 9?
As of iOS 9, Apple requires the certificate for an in-house enterprise app to be explicitly accepted first, before you can use apps from that company.
So, how to proceed ?
First install the Enterprise app you want to use via the Enterprise URL that you received from your IT department.
Then, on your iOS device, navigate to Settings -> General -> Device Management, then tap “Trust” [Company Name].
You now trusted the certificate for the company you just installed the app from. After that, you can open the enterprise apps for that company without problems.