If you are attempting to open apps, you may see an error message that says something like: "AppName.app" can’t be opened because it was not downloaded from the App Store.

To fix the error message "App.app can't be opened because it was not downloaded from the App Store" on Mac, follow these steps:

Go to the Apple menu in the top left corner of your screen and select "System Preferences."

In the System Preferences window, click on "Security & Privacy."

Select the "General" tab.
Under the "Allow apps downloaded from" section, you may see different options. By default, macOS allows apps to be downloaded from the App Store and identified developers.
To allow the app to run, click on the lock icon at the bottom left corner and enter your administrator password if prompted.

Now, choose one of the following options:
If you want to allow apps downloaded from anywhere, select "App Store and identified developers."
If you want to restrict app downloads to only those from the App Store and identified developers, select "App Store."
After selecting your preferred option, close the System Preferences window.
Try opening the app again. It should now run without displaying the error message.

By adjusting your Mac security settings, you have resolved the issue related to running apps that were not downloaded from the App Store.