Let me quote Oracle documentation:

Oracle Error :: CRS-01636
The CSS daemon was started in exclusive mode but found an active CSS daemon on node string and is terminating; details at string in string

Cause
The CSS daemon was started in exclusive mode, which requires that the clusterware stack is down on all other nodes to ensure data integrity. A CSS daemon was detected on another node, so the CSS daemon on this node is terminating.

Action
Stop the Oracle clusterware stack that is running on the indicated node.