The most common reason for this failure is that a tunnel with the same name already exists. The tunnel may have been created by another client connected to the same server.

If the system has several Secure Shell users, they may already have reserved several available ports. In this case, try again to find a free port.

Another possible reason is that you have no permission to open the requested port. The system administrator may have set a policy that restricts opening of communications ports - this is common practice especially with incoming ports. Check the local policy from the system administrator. Note that only the system administrator (root) can open port numbers below 1024.