Hi @gaurav khannaโ , this may happen if you do not have appropriate level permissions on the cluster or you may only have 'Can read' on a notebook that doesn't allow detach and reattach.
https://docs.databricks.com/security/access-control/cluster-acl.html#cluster-level-permissions
https://docs.databricks.com/security/access-control/workspace-acl.html#notebook-permissions
In such cases, you can ask your admin to grant you the necessary permissions.
@Yuanyue Liuโ When you create a new notebook/cluster you get the owner access with 'Can Manage'. permissions by default.
If you still get an error where the notebook doesn't attach to the cluster - in such cases, it must be that your cluster is unhealthy or your driver has crashed. You can try restarting your cluster then.