, You can't create snapshots (or any FTT=1 policy vmdk) because you have a serious issue in your cluster.
I would advise starting with validating in Skyline Health (Cluster > Monitor > vSAN > Skyline Health) that you don't have issues on the Witness e.g. that it isn't isolated from the cluster or that the disks have operation health alert (e.g. disks are PDL or otherwise unusable).
If that is all green then validate from the Witness side the state of the disks and cluster membership e.g. that it is joined to the cluster, it is not in Maintenance Mode and all vSAN disks are present and mounted.
A common cause of Witnesses disks going offline is people taking backup-based snapshots on the Witness VM, this is totally unsupported but usually the disks will remount after power-cycle of the VM (but that being said, I have seen irreparably damage Witnesses and a new one needs to be deployed).