do I really have to buy rpg maker?
Isn't there another way?
Viewing post in Gods of Lewdity (Lust Doll Classic mod) comments
You don't need to buy RPG Maker to play RPG Maker games, or install mods for them.
Lust Doll+ is free with an option to support it's Dev.
If you want to install a mod, you shouldn't need RPG Maker, unless you're installing conflicting mods.
But if you want to create a mod, then yes, you do need a copy of the RPG Maker used to create the game, and the game's project files.
Bringing this up since OP didn't clarify if they were installing or creating a mod.