Kurt Besig:
Also be sure to un-comment the ControlPort and assign an ip to it if different that default in your /etc/torrc.
Alexander Dietrich:
Does the "debian-tor" group have access to the file "/var/run/tor/control.authcookie"?
Thanks, Kurt & Alexander. I had not enabled the ControlPort. That should have been obvious from the error, but I assumed since arm was working for user debian-tor that the ControlPort must already be enabled. After enabling it and giving the cookie file g+r, arm is working as my regular user.