I had this exact same problem and solved it by doing the following:
1) Open terminal ( you can do this using command + space and typing terminal into the spotlight search)
2) run the following command:
sudo mv /Volumes/Macintosh\ HD/Library/Updates /Volumes/Macintosh\ HD/Library/Updates_bk
This creates a backup of your Updates directory which contains a corrupted file and breaks your app store. Since the app store can no longer find the Updates directory it is forced to create a new clean one. You could also just remove the directory but it's better to keep a backup in my opinion.
3) restart computer
Hope this helps!