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Are you sure you put it in the folder correctly? The folder containing the map files should be just next to the Raise Up Your Bat example map, so for example, the path to the midi file should be ...Custom Maps\Map Name/Map Name.mid.
Also make sure the file names are all exactly the same as the folder's name. Otherwise it won't load (I'll try to make it not matter in a future update, but for now it does)
If that's already correct, you could try running it from command prompt, and see if it outputs any errors when it crashes. Ignore the 3 red extension errors, and 1 yellow icon warning, they're known and shouldn't ever cause it to crash. But any extra ones could explain the cause.

Create a new text file inside the folder you extracted the game in, name it however you want, then open it and paste this:

"Deltarune Lightners Live Plus.exe" --rendering-driver opengl3

change the file extension from ".txt" to ".bat".

Now open the ".bat" file and enjoy the game :)