What does the error "Cannot locate authentication server" mean?

This error indicates a connection problem in the PC preventing it from connecting to Zello.  The most likely cause is a firewall blocking access to the Internet or Zello.  The firewall may be a personal firewall on the PC or a corporate network firewall.  If there are no firewalls, it may be due to the Internet Service Provider (ISP) intentionally blocking access to Zello.  If an ISP blockage is suspected, installing a VPN app may help to bypass the block.