RDS Failover API Issue

When I am performing failover_db_cluster operation using python boto3 api by passing writer DB instance identifier , it is giving me below error . But I can able to successfullly use other api like describe_db_clusters .

NOTE : I am running the code from EC2 instances.

Below sample code -

import boto3
session = boto3.Session()
client = session.client('rds')
response = client.failover_db_cluster(DBClusterIdentifier="test-instance-2")
print(response)

botocore.errorfactory.DBClusterNotFoundFault: An error occurred (DBClusterNotFoundFault) when calling the FailoverDBCluster operation: The source cluster could not be found or cannot be accessed: test-instance-2

can anybody let me know why I am getting the issue and what should be resolutions?