It has nothing to do with your saves. It has everything to do with exactly what it's telling you: "NOT installed correctly".
The proper installation process is two steps, and
only these two (
for PC in English):
- Install a fresh/unadulterated version of the VN (no hackery, no shenanigans, just the plain VN). You can do this in a new folder if you wish to retain your existing installation for whatever reason.
- Install the mod as directed in the OP (place all of the contents of the mod's zip inside the "/game" folder of the VN).
That's it,
don't do anything else but start the VN and it will work 100% every time. Try that and report back please.