[mothered]

Why are you trying to partition the drive?

Just allocate the VHD's capacity during the creation of the VM, do the same with the Ram, then mount the ISO and Install the OS thereafter.

look at here on this guide where I have to install Blackarch.
https://blackarch.org/blackarch-install.html

I see.

I was under the Impression you wanted to partition the actual VHD prior to the Installation of the OS. Here's a very good tutorial on how to Install Arch Linux on VirtualBox. It guides you from the Initial setup of the VM, right through to the completion of the OS.