What does transport endpoint is not connected mean?

I am still trying to get transparent proxying working to no avail.

I am doing network diagnostics on client/server and that error is the only thing I have managed to get to show up on mitmproxy output when I ran

 # netcat -z -n -v 80

from the client.

# netcat -u 80

didn’t work at all and just hung.

So what does the endpoint error mean? could it get me closer to solving this infernal mystery of why I can’t get transparent proxying working?