The antiX / MX installer refuses to install to the drive which was used to boot the live system.
This behavior is intended as a safety measure but, because it is inflexible, can be an annoyance.
In the linked page, tradonal has explained a workaround.
For the sake of accuracy, please clarify:
Does the installer refuse to target the entire boot DEVICE (e.g. sda) or only the specific currently-booted PARTITION (e.g sda1) ?