Have you extracted the zip file? There is an installation guide in the file
But simply extract the zip then Copy The PATCH folder into your game folder as the screenshot inside the instruction file
i have windows, so i just copied the PATCH file from the DLC directly into the bewitching-sinner-visual-novel file. so bewitching-sinners-visual-novel -> BewitchingSinners-1.9.8-pc -> "game" -> copied the "PATCH" into that one, then i went out of the "game" folder and opened the BewitchingSinners from the 1.9.8-pc file, the program one, and it worked lol