Have you tried running the tabcmd login command (with relevant options) before running tabcmd refreshextracts? Most tabcmd commands require logging in and connecting to your Tableau Server first.
I know this post is super old, but I'm now running into this exact issue. I have a PowerShell script that had been working great, but for some reason it started giving me that same error. I can bypass the error by including --server 'server name' --username 'username' and --password 'password' in my refreshextracts command, but the extract still fails to refresh. I'm hoping someone has some insight into this. My script logs into the Tableau server before attempting to do any refreshes.