If you experience this while trying to resize your VBox-vdi file / for example so:

root@virtualbox:/home/vbox/VirtualBox VMs/OpenHAB # VBoxManage modifyhd OpenHAB.vdi --resize 204800
0%...
Progress state: VBOX_E_NOT_SUPPORTED
VBoxManage: error: Resize hard disk operation for this format is not implemented yet!

then my friend you tapped in the same rookie mistake i fell and have a Snapshot of the VBox layin around.

check with

VBoxManage showhdinfo OpenHAB.vdi

Please delete all Snapshots and then retry.

vboxmanage snapshot OpenHab.vdi delete UUID

Or use phpVirtualbox…

And when you are there you may need to know this: https://blog.vbonhomme.fr/extend-a-lvm-partition-after-increasing-its-virtual-disk-on-virtualbox/