Had a quick look at your model. You seem to have made very little of the geometry (faces and edges) into groups or components, especially in the main walls, roof, ceilings and windows. That causes great difficulty when you try to edit it.
Try selecting each face in turn with a double click (which includes the edges), then making the selection into a component (press letter G) or group (menu Edit/Make Group). Make sure if you used Component that you check the box that says ‘Replace selection with component’ (it isn’t checked by default if you have other ‘loose geometry’ attached to the selection, which in your model you mostly will have).
Other problems I see in the drawing:
The white roof section is not flat - there’s a diagonal line which if removed, deletes the face that should be there.The ridge line is higher at the open ‘front’ than at the back. But since your near end wall is at an angle to the axes (in plan), you won’t easily be able to have both roof sections in a single plane each. Hard to build that!
Most parts of your walls, windows, and roof have no thickness - only a thin outer shell. Try re-modelling them using the PushPull tool to give them ‘real’ substance.
Your model is quite some distance from the origin, and you have the (irrelevant) default person in your drawing near the origin. Delete the person, then select your whole model and move one corner to the origin.
It would repay you to make the effort either to draw the model again ‘from scratch’ making each element into a group or component as you go, or (not easy - I’ve tried) to convert the existing model, a face or element at a time into groups and/or components.
PS. I attach some notes I made last year for a colleague who was learning Sketchup. You may find them helpful, particularly the use of shortcuts (built in and your own customised ones), and the process of making drawing elements (a) correspond to real world ‘things’ like wall, floor, ceiling, or window frame and (b) create each element as a component or group as you draw it, so its geometry doesn’t get tangled with other elements, as is the case in most of your drawing. ALWAYS, ALWAYS, draw original geometry on Layer 0, then make it immediately into a group or component (components are quicker using shortcut key G).
Using Sketchup.pdf (31.2 KB)