what does that mean and how do I fix it so I can install Windows?
It means you need space on your disk in (probably (X32) NTFS 32). an MS-Win NTFS formatted primary partition(space e.g. 30Gb 150Gb whatever) to aim at. There was not a suitable space on the disk formatted as a "primary partition "C drive" of type NTFS".

Get the latest Debian, it worked in both Gnome and KDE (I've never met a linux distro actually operated properly in Gnome , but i've always had a small quantity of RAM for the "era / generation" at any time).