This error means that agent was not able to establish connection to tracker server. Agent will be attempting to connect to the tracker server which is defined in Profile, parameter "Custom trackers". If you don't have anything there, agents will be connecting to the tracker installed on the same computer as Management Console. Management Console v2.2. and newer gets the tracker installed automatically beside. This tracker gets the same IP address as Management Console.
Default tracker's listening port is 3000.
To fix: check that both outgoing and incoming connection between tracker and agent is possible and is not blocked in firewalls, NAT and router rules between them, both UDP and TCP protocols.
Use tracker option in Profile if you don't need to use tracker.