Native Windows in VirtualBox - No Windows boot files

So I’m following this walkthrough to use my Windows partition in a VM. Though (after hours of pulling out my hair) I’m finally on the last step “Configuring the virtual UEFI firmware and creating Windows boot files”, but there’s another problem. After exiting DISKPART and trying to install the Windows boot loader into the EFI system partition, I find that there’s no D:\Windows\System32 in the ISO (D:) and can otherwise not find the boot files.

The Windows iso is from Microsoft.