The available resources in this cluster are not enough to allow failover to operate for all running VMs.
The cluster does not have sufficient resources to allow for High Availability of all virtual machines.
Check that all hosts in the cluster are available. Check for new deployments of high-spec VMs, or reconfiguration of existing VMs with more resources (cpu/memory). Check metrics such as memoryPressure in the vSphere host Host Memory Overcommit Analysis performance dashboard that show the level of overcommitment for memory resources.
For more information check this VMware KB article.