I have a Proxmox server version 5.1-35 (kernel: 4.13.4-1-pve) and can't create a container snapshot, the button shows greyed out.
I do can create VMs snapshots. This problem also happens on another Proxmox server version 4.4-13/7ea56165. Maybe this is a new change or simple you cannot create a container snapshot (I say this because cannot find much reference about this).
Same problem:
https://forum.proxmox.com/threads/ve-4-4-cannot-create-a-container-snapshot.35570/
I do can create VMs snapshots. This problem also happens on another Proxmox server version 4.4-13/7ea56165. Maybe this is a new change or simple you cannot create a container snapshot (I say this because cannot find much reference about this).
Same problem:
https://forum.proxmox.com/threads/ve-4-4-cannot-create-a-container-snapshot.35570/