Hi Guys

I am following the documentation from https://www.kali.org/docs/usb/usb-standalone-encrypted/

everything was nice and perfect (of course corrected some mistypes like)

sgdisk --change-name=1:/boot --change-name=2:GRUB --change-name=3:EFI-SP 00change-name=4:swap --change-name=5:rootfs /dev/sda


but once reached to the section of Btrfs Adjustments
I am completely stuck.

the doc states that the debian installer would already create subvolume @rootfs
the issues are already starting that I can't create subvolumes @, @home, @root and @snapshots as these already exist.
Next, unmounting /mnt/point and /target/boot/efi (yeah.. my equipment no longer provides any legacy options. EFI or nothing) worked
but unmounting /target is throwing that the device is busy (even if every single step of the documentation went absolutely fine and nothing was altered)

At this stage decided to force unmount the /target and continued, but at the point of mounting
Code:
mount -o subvol=@rootfs /dev/mapper/LUKS_ROOT /mnt/point
I am getting the error that I have an invalid argument ...
That means I am unable to copy the fstab from the mount to the target/etc/fstab and amending the UUID wouldn't work (the fstab which I would be later editing does not even contain @rootfs)

Would someone be so kind and point me into a direction to make it working?
So far till now, I was using a live USB with persistant encrypted storage, but once I have found this article as a documenation, I have even ordered myself M2 128GB drive just to place a full Kali onto it