This issue occurs when high availability (HA) strict admission control is enabled and only one ESX/ESXi host becomes available in the HA cluster once the ESX host upgrade begins.
There are two ways to configure the HA cluster to allow for the ESX/ESXi host to enter maintenance mode:
By disabling strict admission control:
- Right-click the cluster and click Edit Settings.
- Under Cluster Features, select VMware HA.
- Under Admission Control, select Disable: Power on VMs that violate availability constraints.
- Click OK.
By temporarily disabling VMware HA in the cluster settings:
- Right-click the cluster and click Edit Settings.
- Under Cluster Features, select Turn off VMware HA.
- Click OK.