What user are you logged in as? “user”, “root”, or “sysmaint”?
In dom0 terminal you should be opening your Whonix TemplateVM terminal, or Whonix AppVM Terminal using this syntax:
qvm-run -u root Whonix-TemplateVM-Name xfce4-terminal
or
qvm-run -u root Whonix-AppVM-Name xfce4-terminal
Try that and you should be logged in as root, and be able to run commands.