In Apple’s OS X Operating System (now called macOS), there is a security feature called Gatekeeper in the System Preferences area to help keep out rogue programs out of your desktop and laptop system. To change its status from enabled or disabled, you need to execute below in the Terminal (which is PC’s version of the Command Prompt) the following command.
sudo spctl --master-enable; spctl --status; exit
Adding ; exit is optional and can be left off if you need to disable Gatekeeper again or continue use the Terminal.
Once the command is entered, you will need to input your password credentials to authorize the changes. Please be aware that the password will not show for your security. Also, the System Preference windows must be closed when attempting to change the status if you wish to see and verify the changes as it will not refresh while the System Preference window is loaded.