Error response from daemon: rpc error: code = 14 desc = grpc: the connection is unavailable

I have two servers in Docker Swarm, but when I need to add a third server - I get the result:
docker-machine加入节点时出现错误：
Error response from daemon: rpc error: code = 14 desc = grpc: the connection is unavailable
All servers in one network.
所有服务处于同意网络
What could be the problem?
问题是？
From official Docker swarm tutorial
官方文档，对于swarm之旅：
The following ports must be open on your docker hosts.
docker主机之间下面端口应该打开
**TCP port 2377 for cluster management communications
TCP and UDP port 7946 for communication among nodes
UDP port 4789 for overlay network traffic**

To enable this ports run the below command on all your docker hosts. kindly follow the digitalocen article for complete steps.
可以在所有docker hosts上运行以下命令，请参照完整的digitalocen完整步骤

firewall-cmd --add-port=2376/tcp --permanentfirewall-cmd --add-port=2377/tcp --permanentfirewall-cmd --add-port=7946/tcp --permanentfirewall-cmd --add-port=7946/udp --permanentfirewall-cmd --add-port=4789/udp --permanent