This error means that the agent was not able to establish network tunnel with any other agent in this job. For files to be synced, agents must be connected to each other directly either via TCP or UDP.
Here's how tunnel is established and connectivity is resolved. The reason the error may appear at any step below:
1) Agents cannot connect to tracker servers.
Tracker is necessary for agents to learn addresses of each other (role of tracker). By default tracker server is installed on same computer as Management Console, uses port 3000 and all agents try to connect to it right away. Else, they connect to the tracker defineD in "Custom tracker" option in Profile.
Check that connection to tracker is possible, TCP and UDP in and out. Install tracker in another location outside LAN and get agents connect to it.
2) Agents don't connect to each other.
Once they connect to tracker and learn each others' addresses -public and local - they'll be trying to establish tunnel on these, both TCP and UDP protocols will be attempted.
Open and forward agents ports (3839 by default on all possible NATs, routers and firewalls between agents.
If nothing helps, send the debug logs from the errorred agent and any other online agent from this job to support.