I was also looking ! It seems to be a simple issue with 'FixNum' being deprecated in the newer Ruby version, replaced by 'Integer'.
I'll edit the file and publish an updated version asap.
Under the author's notice it's allowed...

You seemingly haven't installed it from its RBZ.
If you simply copy the ../Plugins/HouseBuilder_extension.RB file over to the newer SketchUp version it misses the images to make the toolbar-buttons [and worse!]...
Unfortunately the original author was in the dim past [20years ago] and made a half-baked extension loader file, and he didn't give his subfolder the same name as the RB file.
The new[ish] auto-migration tool assumes this file/folder naming convention and so it misses out copying the subfolder of images [and some other vital support files that are in that folder].
To fix it you need to find the whole HouseBuilder folder [and its contents intact] in your older version's Plugins folder and copy that whole folder over into the newer SketchUp version's Plugins folder.
When you startup SketchUp it should now find the subfolder and load the buttons' images and the other vital support files...

Perhaps the best Fredo idea yet !

It's now available as an RBZ download from our PluginStore or through the SketchUcation > ExtensionStore dialog within SketchUp.
Well done.

@mihais

I see you're now listed as a developer, so you just need to do the publishing steps...

Thanks for this.
I look forward to it in our PluginStore / ExtensionStore shortly...

@Ramon3D

Several questions...

Does each of your students have a full copy of the the extension installed on their computer in their user's SketchUp Plugins folder ?
Also do they have a copy of the LibFredo6 installed on their computer in their user's SketchUp Plugins folder ? It's needed to support the extension.
And do they have a copy of the SketchUcation toolset installed on their computer in their user's SketchUp Plugins folder ? That's needed for the licensing.
Do they have a copy of the relevant license file saved onto their computer, and also has it been validated for that user and that device ?

@fredo6 will probably reply in more detail about the multi-seat licensing setup.

Following on from the previous 'usage' post, I've been asked about how you can export a Scene-tab containing the actual shadow reported with its area and %.
My other tool TIG_ShadowProjector will do this.
https://sketchucation.com/pluginstore?pln=TIG_shadowProjector
Read its thread for usage notes and guidance.
You need to set up a Plan [or Plan-like] view and model any building volumes simply to avoid excessive checking of objects that are ultimately found not to project meaningful shadows - e.g. internal floors, partitions and fittings, then faces for the neighboring land.
Select the face[s] to receive the shadows... Run the tool and choose the options.
The projected shadow can be represented in various ways -
probably the best for your purposes is the 'Faces[Edged]%-sq' - it draws each shadow as a 'gray' face, with edges visible, and with a text-label at its center of the 'Shadow' group showing the % and area [uses the current model's area units - sq' or sqm] of that face that's shaded at that date/time.

Following my post about setting up your model's shadows for date/time-stamping and export, I had assumed the first steps to be self-evident - but for the avoidance of doubt...
Your model needs to be Geo-located to show correct shadows - because the UTC Time-Zone and its actual location are both important.
Use the Model Info section for this.
For example, the shadows at 08.00 in Southern England and in Northern Scotland are quite different despite them sharing a Time-Zone.
Also the model's default 'North' is the Y Green axis, but there might be an offset shown orange in the results. Make sure the Z Blue axis is 'up'.
If you have modeled the building 'on axis' but have a building that is in reality 'off axis', you can resolve this by making a group of everything, and then rotating that group to suit the actual North, when you edit that group its axes with it will still align as before; any further modeling within that group will also remain 'on axis' for your building, whilst it is actually placed in the world correctly.
The model's Shadow settings should now reflect its Geo-location.
Alternatively of older versions, if you want to adjust the 'north solar axis' manually, then there's a su_ tool available in the EWH - https://extensions.sketchup.com/extension/393f5153-ba5f-4f5f-849f-46b32cf64bd4/solar-north
I have Geo-location set up on my usual Template for my regional location, set manually using the Model Info dialog's section.
You can set up templates for various locations if you work in several different areas.