After El Capitan update, I cannot sign in to anything with my Apple ID: '"This action could not be completed. Try again"
Solution 1:
Execute the following commands in Terminal:
sudo mkdir -p /Users/Shared
sudo chown root:wheel /Users/Shared
sudo chmod -R 1777 /Users/Shared
Solution 2:
I ended up updating iTunes then signing in, after the update completed and a restart. Once the iTunes sign in was successful everything else was able to sign in. Hope that helps someone.