This is going to be a quick fix. I've encountered a few clients that have a rare problem where they try to log into their iCloud (via system preferences) or log into the App Store and they are greeted with an error stating "This action cannot be completed. Try again."
You have bad system permissions. Why? Looks like it's associated with an OS update.
Fix: Open terminal and paste in each line separately (3 lines), hit enter after each one, then enter your password. Line two and three wont prompt for your password after you've entered it into the first line.
sudo mkdir -p /Users/Shared
sudo chown root:wheel /Users/Shared
sudo chmod -R 1777 /Users/Shared
Restart your mac. iCloud and App Store will work. You're welcome.