Under “Allow apps to be downloaded from”, select App Store and identified developers. Once this is done, you’ll be able to open most of the apps that aren’t on the App Store. However, you still won’t be able to open apps that aren’t recognized by macOS. Setting your Mac to open apps from unidentified developers. Mac app icon maker. If you want your Mac. Change Allow apps downloaded from: to App Store and identified developers. Note: For Mac High Sierra (10.13.x), you will need to click on Allow too. Click the lock icon again to prevent any further changes. Installing the Zoom application. Visit our Download Center. Under Zoom Client for Meetings, click Download. Double click the downloaded file.
Click Allow From Anywhere when prompted. Doing this will allow you to install software from unsigned developers without having to confirm from within this menu. If you don't install any unsigned software within 30 days, you'll have to re-enable this settings. Click the lock icon if you'd like to prevent further changes. Advanced Mac users may wish to allow a third option, which is the ability to open and allow apps downloaded from anywhere in MacOS Catalina, macOS Sierra, macOS High Sierra, and MacOS Mojave. To be clear, the “Allow applications downloaded from anywhere” option is hidden by default in Gatekeeper for macOS from Sierra onward.
Sign out of App Store.
Sign out of iTunes.
Sign into the App Store.
Sign into iTunes.
Accept the iTunes Terms.
Close both App Store and iTunes.
Open Terminal
Run the following command: sudo find /private/var/folders -type d -name com.apple.appstore -exec rm -rf {} ;
Input password when prompted.
Run this command to reboot your mac: sudo reboot
After reboot open App Store and try to purchase a app.
Allow Any App Install Mac Mojave
That fixed the issue for me, I hope it does the same for you :-)