I assume you mean this game?
https://jevilhumor.itch.io/undertale-engine
If so, note that the game is a NON-executable file that requires a previous application.
REQUIREMENTS:
A computer running either Windows or Mac OS
A copy of Game Maker Studio 2.x
A Game Maker Studio 2 License (there is a free trial, but we prefer you use "Desktop", only $99.99 retail. DO NOT USE CRACKED VERSIONS, CAN CORRUPT YOUR BUILDS/LOSE PROGRESS)
The latest version of the engine
A basic understanding of GML (Game Maker Language)
The Itch application can only install games that have a valid executable for the operating system from which it is run and since this is not the case, it does not appear.