Apple - iOS Untrusted Developer error when testing app

Enterprise and other certificates have to be manually trusted on iOS 9 when they didn't have to on iOS 8 and earlier.

• Open the Settings app
• Open General
• Open Profiles (you won’t see this until after the first profile is installed on an iOS device)
• Choose the affected profile and trust it

If you don't see "Profiles & Device Management" then your device needs to reinstall the app or get the appropriate profile from MDM / other enrollment. Also, if your network or system blocks any outbound traffic to Apple servers for time sync, OCSP/CRL or notarization checks, you will see errors or potentially have other challenges to run apps.

Alternative way to fix this error:

• Open Xcode app on your Mac
• Navigate to Window > Devices
• Right click on your device > "Show Provisioning Profiles..." > and delete the profiles
• Delete the installed app on your device
• Now rebuild and install the app again

Note: In my case only "Settings > "General" > "Profiles" was shown instead of "Profiles & Device Management", and these steps solved the issue.