Couchbase Fail Over Options

After Couch base Install Graceful Fail Over option is disabled . How to fix this ?

Attention: The graceful failover option is not available either because the node is unreachable or replica vbuckets cannot be activated gracefully.

Warning: Failing over the node will remove it from the cluster and activate a replica. Operations currently in flight and not yet replicated, will be lost. Rebalancing will be required to add the node back into the cluster. Consider using “Remove” and rebalancing instead of Failover, to avoid any loss of data.!