Not sure why, uninstall all instances of Tableau and re-install and this time activate using the product key when prompted.
Each desktop key will work in two machines. first check the key already used in two machines and if not used then give to user and add key.
The Issue was resolved.
1, I make sure the Licence is Valid and not assigned to anybody else
2,I asked user to Refresh the Key under Help > Manage Product Keys and restart Tableau Desktop.