The Clusters - Node Unavailable Alarm alarm is raised when Spotlight detects that at least one cluster node (server) is not currently running as part of the cluster.
This alarm can only be raised when you are connected to a SQL Server that is running as part of a Microsoft Cluster Server (MSCS).
When a cluster node (i.e. a server that participates in a cluster) is unavailable, the workload that it normally processes will fail over to other servers in the cluster, resulting in lower performance.
When this alarm is active you should:
Spotlight on SQL Server Alarms (page 1)