Why do I keep getting this message in the app store:"We could not complete your purchase" 10.13.3

Mac OS High Sierra 10.13.3 (17D102) iMac (27-inch, Late 2013)

Trying to purchase any app or download any free app from the app store give me this message:

"We could not complete your purchase. The product distribution file could not be verified. It may be damaged or was not signed."

I have cleared the cache by doing this:

* Go to Finder and press ⌘ + Shift + G;

* Locate ~/Library/Caches/com.apple.appstore and delete com.apple.appstore which are cache files;

* Then press again those commands and go /private/var/folders, open each folder and each subfolder until you find com.apple.appstore and delete this folder;

* Restart your Mac.

I have even booted in safe mode and done the same per someones suggestion on here, but the same thing keeps happening.

Any thoughts?