After discussing this in a chat and debugged the issue, it turned out that the required directive PermitTunnel yes was not in place and active. After adding the directive to /etc/ssh/sshd_config and reloading sshd by service sshd reload this was resolved.
We added -v to the ssh command to get some debugging information and from that we found:
debug1: forking to background root@ubuntu:~# debug1: Entering interactive session. debug1: Remote: Server has rejected tunnel device forwarding channel 0: open failed: administratively prohibited: open failed debug1: channel 0: free: tun, nchannels 1
The server actively rejected the tunnel request which pointed us to the right directive.
rootaccount as stated in the tutorial: 'from the root account of your local server'?rootaccount and commands are sent from a#