Error message appears for an agent in Sync job. It means that the shared folder is missing from computer.
The error appears after the job is configured, if the agent, in the course of syncing, does not find the share in its place. Check job's info and see share location for the agent or its group. Then check agent's drive and see if the folder is indeed located there.
Note: if folder is moved to another location, agent won't follow it.
1) restart the agent. It will create new share as per job's settings. Files will be re-downloaded from other agents.
2) edit the job and point the agent to another folder. Use this option if the folder was moved to another location on computer, or if you want agent to sync a different folder. Once reconfigured, the agent will scan the folder, build folder tree and merge it with that on other agents. When merging, agents will compare files' hashes. Files with matching hashes won't be re-synced.