I'm trying to set up the starting area on a Race for Wool map, but every time I type in /zone -s, I get the error given above the "No valid team names given".

I've named the teams Right and Left in the XML so any help would be appreciated.

The command for setting the start region is /zone -S (note the capital S).

To setup a zone for a specific team, the command is /zone <teamname>.