Before adding or removing the witness host in a stretched cluster, all current hosts must be connected. If a current host is disconnected, you cannot add or remove the witness host.
When you add or remove a witness host in a stretched cluster in which some hosts are disconnected, the operation fails with an error status: The operation is not allowed in the current state. Not all hosts in the cluster are connected to Virtual Center.
When the witness host joins or leaves a stretched cluster, vSAN must update the configuration on all hosts in the cluster. If one or more hosts are disconnected from the vCenter Server, the witness host cannot be added or removed.
Verify all hosts are connected to vCenter Server, and retry the operation. If you cannot rejoin the disconnected host, remove the disconnected host from the cluster, and then you can add or remove the witness host.