Fail to install Kali with BTRFS

I have been trying to install Kali with BTRFS using the excellent documentation for it. Unfortunately repeated attempt failed for the following reasons I think:


On step 21:
failed to "umount /target" with a resource busy message.

I then tried to ignore and continue,

but then on step 26:
failed to "mount -t btrfs -o subvol=@root /dev/sda2 /target/mnt/root" with a "No such file or directory", yet they do exist!

The system is a dual boot system with a Debian derivative on the other disk and a UEFI boot.

The Kali install works but, of course, BTRFS is not a expected!

Kali is the latest installer downloaded yesterday. The only variance from the docs is that I choose KDE instead of default XFCE.

Any guidance, or must I abandon BTRFS which was one of the reasons, not the only one, to switch to Kali?

Any more info required?

Thanks for any help.