My guess is that you tried installing the mod on the new version 1.0a release. Whereas this mod designed to work for version 0.5.25. I've just uploaded a different version of the mod which is compatible with v1.0a.
If that isn't what is causing your issue, I need more info. What version of the game are you running? Are you using other mods?
Aric included a bunch of fixes from the bugfix patch. The modification system itself has been changed quite a bit. I think the version of the bugfix Aric included contained some core changes to the rules for mods. (The relevant change is how numeric indexes are used)
In my mod, open the 'scripts/Mansion.gd' file in any text editor. The first line of text has the number 32 written twice. Change both of those numbers to 31 and then try to install it.
