This error can be resolved by changing the security settings of your Mac.
To disable the security settings and open the application, follow the steps below.
▼ Method 1
Go to "Preferences" > "Security & Privacy" > "General" > Execute permission for the app you downloaded:
"App can't be opened because it is from an unidentified developer" > "Open anyway" > Open the app.
▼ Method 2
① Search for the application you want to open by using the Finder .
* Do not use the Launchpad. The shortcut menu is not accessible with the Launchpad.
② Control-click on the application icon and select Open from the shortcut menu.
③ Click "Open".
The application will be saved as an exception to the security settings and from now on you will be able to open it by double-clicking it, as with any other registered application.
Reference (external website): macOS Sierra - Open a Mac app from an unidentified developer