The current guest configuration does not support taking new snapshots is a common problem with Proxmox. Most people don’t know this is easily solved by making sure that the type of storage you use for VM’s is either lvm-thin or ZFS. If you don’t want to take a memory hit, make sure to use lvm-thin. You can also convert existing images to a new disk on the new zfs or lvm-thin storage you setup.
vm is locked is common issue in proxmox that is solved with a simple command to the vm on the command line
qm list qm unlock $VMID
Solves : container is locked proxmox
unlock container proxmox
unlock vm proxmox
vm unlock proxmox