When I first started working with Docker Swarm, I discovered a confusing error that took me some time to resolve. When I’d attempt to make any change to a Swarm Node, I’d receive the error:
rpc error: code = Unknown desc
Even after restarting every node in the Swarm, I wound up with the same error. On a whim, I checked the time on each server, only to discover that node1 and node2 were configured for different time zones. Because of that, the Swarm nodes had trouble communicating with one another. To fix that problem, I logged into the nodes and ran the command:
sudo timedatectl set-timezone ZONE
Where ZONE is the correct timezone for my area which can be found with the command:
cat /etc/timezone
Once you’ve made sure the time zones are correct for each node, that error should go away and you can label your nodes as needed and set their Availability.
from: How to manage a Docker Swarm with Portainer | TechRepublic