How did you create the user? Command line useradd or the GUI?
If using the GUI, what type of user is it? Standard or Administrator?
Try to run the following command in a terminal:
You may need to add the user to adm and sudo group (
usermod -a -G adm username) to be able to shutdown or reboot.
NOTE: If you want to understand why, check /etc/sudoers you see the following:
# Members of the admin group may gain root privileges
%admin ALL=(ALL) ALL
# Allow members of group sudo to execute any command
%sudo ALL=(ALL:ALL) ALL