An administrator receives the alert: What is the most likely cause?

Nutanix clusters are designed for high availability and fault tolerance. A fundamental operational constraint for maintaining this resilience is that only one node can be in maintenance mode at any given time. This policy ensures that the cluster can always withstand at least one subsequent component failure while maintaining data redundancy (e.g., Replication Factor 2). If an administrator attempts to place a second node into maintenance mode while another is already in that state, the system will block the action and generate an alert, as this would violate the cluster's high availability guarantees.

A. While insufficient resources on other nodes can prevent maintenance mode entry, the "one node at a time" rule is a more fundamental, absolute prerequisite that is checked first.
C. The alert states the node "cannot enter" maintenance mode, which means the action failed. If the node were already in this state, the action would be unnecessary or would not fail in this manner.
D. The resource availability of the node entering maintenance is irrelevant. The critical resource check is performed on the remaining nodes that must host the migrated virtual machines.

1. Nutanix Prism Web Console Guide (AOS 6.5): In the section Hardware Management > Node Management > Node Maintenance (Prism Element), the documentation explicitly states, "You can place only one node at a time in maintenance mode." This confirms the operational constraint.

2. Nutanix University, Nutanix Cluster Administration (NCA) v6.5 Courseware: The Module 4: Managing a Nutanix Cluster lesson on Node Management emphasizes that for cluster health and data availability, maintenance operations are serialized, allowing only a single node to be in maintenance mode simultaneously.

3. Nutanix Acropolis Advanced Administration Guide: The guide details the pre-checks performed before a node enters maintenance mode. The check for other nodes already in maintenance is a primary validation step before resource availability calculations are performed.