There is a bug with the GUI installer Cnchi when the target partition is in a LVM. If the physical volume’ name contains ‘-’, it is replaced by ‘- -’ in /dev/mapper, but Cnchi doesn’t handle that.
For example, if I have a physical volume named “laptop-anthony”, with a logicial volume “root”. The volume will be in “/dev/mapper/laptop- -anthony-root”, but Cnchi will try to mount “/dev/mapper/laptop-anthony-root” (because it certainly concatenates the physical volume name and the logical volume name).
Edit: The difference between - and – is not very clear with this forum CSS, so I replaced ‘–’ by ‘- -’.