When I tested at the end of the course for me the building preview game object was always disabled. After some time I figured out that in the Update method
if(buildingPreviewInstance == null) { return; }
UpdateBuildingPreview();
Was always null so the UpdateBuildingPreview method wasn’t running.
What solved my issue and its because I’m using Unity 2020 (I think) is moving it from Update into LateUpdate.
I thought I’d leave this here in case someone else runs into this issue.