Is it possible that there may be a firewall or proxy that could be rejecting the request? Have you tried the command on any other machines or through any other networks to rule out any network connectivity issues?
Senior Technical Lead
Thank you for the suggestion! We were able to work with our security team and find that a firewall was in-fact blocking direct connections to Tableau Online. Once we white listed all *online.tableau.com URLs, the tabcmd login command was successful.