Huh, that's strange, I just tested the script in-editor again and it seems to be working as expected... Are you using an outdated version of STM? OnRebuildEvent was added in v1.8 so you'd have to be using a pretty old version to see an error like that. If the asset store/itch gave you that as a download, I need to get in contact with them! Otherwise, please try updating STM!