how to delete aws msk cluster under maintainance

It cannot be deleted during maintenance as described in this document.
You must wait until the status becomes active.
https://docs.aws.amazon.com/msk/latest/developerguide/msk-cluster-states.html

Amazon MSK is performing routine maintenance operations on the cluster. Such maintenance operations include security patching. You can still use the cluster to produce and consume data. However, you cannot perform Amazon MSK API or AWS CLI update operations on the cluster until it returns to the ACTIVE state.