How to open apps not from Mac App Store
By default macOS allows you to open apps from the official Mac App Store only. If you have this still set as your default you will be seeing the warning when you try to open an app for the first time.
Luckily you can make a simple change to your settings that will allow you to open some third-party apps that aren’t on the App Store. It won’t mean that you can open every third party app without issue, but it will certainly mean you see fewer warnings.
1) Open System Preferences.
You’ll still be prevented from opening anything macOS doesn’t recognise, but at least you will be able to open apps that weren’t purchased from the App Store.
How to open a blocked app
If you attempt to open an app and macOS stops you from doing so, that doesn’t necessarily mean there is something wrong with the app. But it will indicate that the app isn’t from an ‘identified developer’.