Hi!
This extension was made using the current Monthly release (v2024.14.1), not LTS or Beta. As far as I know, it won’t work on older versions regardless of any new code it might use.
I tried remaking the extension in v2024.13.1, and I got the same “fatal” error as you. However, I tried remaking the extension in v2024.11.0 (with zero changes), and it worked completely fine.
It seems to be an issue with the v2024.13.1 runtime, unfortunately. I went ahead and uploaded the v2024.11.0 yymps to itch.io, but you could also try copying over code from the source .gml file into a script and reformatting it for your own purposes in v2024.13.1.