“App can’t be opened because it is from an unidentified developer” is a common error you will face obviously. The another error is “App is damaged and can’t be opened”.
How to fix these errors in MacOS Sierra?
Here below is “CleanMyMac 3” can’t be opened because it is from an unidentified developer error
From Sierra desktop, Apple menu > System Preferences > Security & Privacy > Select “General” tab > check on “Anywhere” under “Allow applications downloaded from”, you then can launch any app from any location or developer.