SSH: Running `nc -l -p X` results in Error: Couldn't setup listening socket (err=-3)

Hey all!

As in the title. I’m following the instructions here: Mullvad Port Forwarding. Things were working up until Step 4: Test your port forwarding where it told me that Port is unreachable. So, I tried their advice on Test your port forwarding#Linux. I followed the instruction sudo ufw allow X and allowed the port that was given to me by Mullvad. That worked fine, but when I tried to run nc -l -p X, I run into Error: Couldn't setup listening socket (err=-3). I tried running nc -vvv -l -p X, but the only information I get is Error: Couldn't setup listening socket (err=-3). The port is in use, but that’s kind of the point, right?

Am I doing something obviously wrong?