Mac gives fine-grained option to let the users decide what kind of apps can be installed. You can install from mac app-store, identified developers or from anywhere. The option can be accessed under
Security & Privacy in System Preferences.
Mac Sierra has disabled the ability to open Mac apps downloaded from anywhere. It is a good practice but some of the best apps are outside the mac app-store. The option to install from anywhere can be enabled by running the below command in the terminal.
sudo spctl --master-disable
The same can be disabled after installing the required software because it is just good practice.
sudo spctl --master-enable