When you use the Host Failures Cluster Tolerates admission control policy, vSphere HA clusters might become invalid (red) due to insufficient failover resources.
If you select the Host Failures Cluster Tolerates admission control policy and certain problems arise, the cluster turns red.
This problem can arise when hosts in the cluster are disconnected, in maintenance mode, not responding, or have a vSphere HA error. Disconnected and maintenance mode hosts are typically caused by user action. Unresponsive or error-possessing hosts usually result from a more serious problem, for example, hosts or agents have failed or a networking problem exists.
Another possible cause of this problem is if your cluster contains any virtual machines that have much larger memory or CPU reservations than the others. The Host Failures Cluster Tolerates admission control policy is based on the calculation on a slot size consisting of two components, the CPU and memory reservations of a virtual machine. If the calculation of this slot size is skewed by outlier virtual machines, the admission control policy can become too restrictive and result in a red cluster.
Check that all hosts in the cluster are healthy, that is, connected, not in maintenance mode and free of vSphere HA errors. vSphere HA admission control only considers resources from healthy hosts.