For any future Googlers who stumble across this thread like I did while trying to solve this problem!
I got this error when I tried to install the mod on Nuclear Throne v19, the "mods" beta branch on Steam. This mod appears to need the latest version of Nuclear Throne, not the "mods" branch like some other mods do. Open the properties for the game on Steam, go to Betas, and select "none". Then wait for Steam to finish downloading the changes, and reinstall the mod. It worked for me then!