If you’ve seen this error more frequently in recent months: Welcome to macOS Sierra and the future of personal computing. What’s happening here is, that Gatekeeper, a security feature that’s been around since Mac OS X 10.7.5, is a lot more aggressive since Sierra.
Luckily - if you know what you’re doing - there’s an easy way to disable
sudo spctl --master-disable
sudo spctl --master-enable
Simply enter this into the Terminal and you’ll be good to go. In any case, I suggest to re-enable Gatekeeper after you’re done. Better to play by the rules on macOS or your Mac will punish you.