1. Please force update using this command.
rm -f /usr/local/cpanel/Cpanel/rvversion
2. Go to root WHM -> Server Configuration -> Tweak Settings -> System -> “Accounts that can access a cPanel user account”, and mark at "Root, Account-Owner, and cPanel User"
If still not work, please SSH to your server and run following command.
ln -s 5.8.8 5.6.2