The vSAN Healthcheck reveals that one or more vSAN Hosts are in vSAN Decommission State as described in KB 318411
This can be verified via SSH/Putty Session as well:
esxcli vsan cluster get

You can also run the below script from one of the hosts in the cluster to see which hosts in the cluster are in vSAN Decom state.
echo "hostname,decomState,decomJobType";for host in $(cmmds-tool find -t HOSTNAME -f json |grep -B2 Healthy|grep uuid|awk -F \" '{print $4}');do hostName=$(cmmds-tool find -t HOSTNAME -f json -u $host|grep content|awk -F \" '{print $6}');decomInfo=$(cmmds-tool find -t NODE_DECOM_STATE -f json -u $host |grep content|awk '{print $3 $5}'|sed 's/,$//');echo "$hostName,$decomInfo";done|sort
Sample output:
hostname,decomState,decomJobType
esxi-1.example.com,0,0
esxi-2.example.com,0,0
esxi-3.example.com,0,0
Anything other than 0 means there is a host in vSAN Decom State.
hostname,decomState,decomJobType
esxi-1.example.com,0,0
esxi-2.example.com,0,0
esxi-3.example.com,6,0 <---