To fix this issue, you need to enable installation of applications from sources other than the Mac App Store on your macOS.
Users can follow the directions below to prevent this error message from appearing in one of two ways:
- Changing Gatekeeper's settings (10.8.x / 10.9.x / 10.10.x / 10.11.x)—this is useful for users who download a lot of programs that are not from 'verified sources.'
- Changing Gatekeeper's settings (10.12.x / 10.13.x)—the same steps as above, but for Mac's Sierra and High Sierra OS
- Manually allowing individual applications to launch—this is the best method for users who do not want to change the global setting to allow all applications to run.
For more information about this message, please visit Apple's KB article on the topic: http://support.apple.com/kb/HT5290