"you need to have the Enchant library installed" -- README
To build anything you always need the development versions of libraries installed _if your distro separates them_. And different distros call the separated development packages different things, `libenchant-dev` on Debian and derivatives, `enchant-devel` on Fedora and other headgear, and probably other things elsewhere.
So its not a simple "install XXX" so its generally assumed in Geany and plugins that people building the software understand what they are doing and how their distro works.