Hello! Thanks for reaching out.
As stated under the Compatibility section in this page, this mod doesn’t work on non-English versions of Demons Roots. The “splash.js” file that prompted your error is a custom plug-in that Kagura Games, the publisher of Demons Roots in English, uses in some of their releases to display their logo and other information at the start of the game.
That said, if you’d still like to install this mod, I’d be happy to make an alternate version for you. Just tell me what language your version of the game is in, and I should be able to adapt the mod to it.
However, if your game is in English, then I’m afraid you’re playing the unpatched version, which also lacks the “splash.js” file, among many others. If you also need help with that, please don’t hesitate to send me another comment.