App Store apps can't be opened after upgrading macOS 10.12 to 10.13

Ok, I got so sick of it that I decided to reinstall my OS. I even formatted the HD, and I did not restore from any backup. While at it, I moved to 10.14.6.

When that installation was freshly done, the first thing I did was security updates and a reboot.

After that I wen't to the App Store and tried to install the app again. The error from yesterday was gone but to my surprise I got the following error:

Then I followed all the steps here except the Upgrade to macOS Catalina:

https://www.saintlad.com/we-could-not-complete-your-purchase-app-store-on-mojave/

1. Check Network Connectivity
2. Verify Apple ID in App Store & iTunes
3. Accept iTunes New Terms & Conditions
4. Clear App Store Cookies and Cached Files
5. Clear App Store Temporary Files
6. -U-p-g-r-a-d-e- -t-o- -m-a-c-O-S- -C-a-t-a-l-i-n-a-
7. Get Support from Apple

When I was on the phone, I clicked to get the same error as in the picture to make sure I didn't confuse it with yesterday error's message. And yes I still got the error. Later in the conversation he asked me to try another app, which I had done before. But magically it worked now, and suddenly all worked.

So yeah, I recommend people to call Apple, and let them do whatever it is they are doing to fix this problem.

It looks as this is all apart of a higher issue.

App Store apps do not install properly on a fresh install of High Sierra on internal SSD or HDD]
https://discussions.apple.com/thread/252037712?answerId=253880193022&login=true&page=9

App Store apps failing to install on High Sierra
https://talk.tidbits.com/t/app-store-apps-failing-to-install-on-high-sierra/14183

I just happen to see the following.

From a clean reboot, run in terminal “spctl --assess -v /Applications/YourApp.app”

From a Finder window, under Applications, right-click (Ctrl-Click) on your app, and select “Signer Info”

Enjoy your app

I hope it helps.