How to fix / Reset App Store app on Mac OSX - El Capitan
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.