No worries,

Are you sure the user or group is debian_tor?
The default is debian-tor in Ubuntu.

If that isn't the problem,

First I would be sure tor is actually running.
top

The second will show if tor is actually running as the user you think it is.

If it is, then see if it is listening on the control port
sudo netstat -ntlp | grep LISTEN

If it is I would suspect that either a firewall is blocking that port. If you have one running
try shutting it down for a few minutes while you try to start arm.

Or maybe it is a permissions issue where arm is not running as the same user as tor. You could try starting arm as root
to see if it would start. But, do not run arm as root full time. Only try to start it as a test.