Your form is bound to the same recordsource as the first combobox.
The details following the second combobox are set to a different table.
A form can only have one recordsource at any one time. So, the fields under the second combobox are trying to reference fields in the form's current recordsource (the stuff under combobox one) but can't find them - so, it's telling you it can't find fields in the Commercial Clients table with the names: Extension, H_Floor, and H_Room.