Thank you. I’ve had a quick look.
Your model is almost entirely ‘loose geometry’ with only one group to separate the remain ‘plan’ part from what you have drawn.
You need to learn more of the basics of modelling, particularly using groups, components and tags - see for example learn.sketchup.com and the YouTube video series Line Tool - Square One and subsequent tools in the series.
Generally speaking, it is a BAD idea to assign tags to edges and faces - DON’T.
Instead group your geometry or (better) make it into components as you go, and assign tags only to groups or components - leave edges and faces Untagged, and set (and LEAVE) the ‘pencil icon’ in the Tags panel at Untagged.
How did you draw this part of your model?
I’d have expected a single arc for the bottom front edge, and for the outer edge of the plan at the front. Instead, they are a mixture of short arcs and loose edges, of different apparent radii.
Your curves don’t seem to be any particular radius - at least, they aren’t round numbers in metres or sometimes even centimetres.
Is it deliberate that the straight edges at either end of the plan arc are not square to the front edge? The blue highlighted edges are at right angles to the front edge:
I’ve redrawn part of the model for you, using true arcs for the curves, all concentric about the midpoint of the opening in the floor, made a few components of the redrawn pieces, and uploaded it here.
I’ve also draw verticals from which you can draw the sloping side walls. They are just equally spaced over the difference in height between the bottom and top corners.
Wilkie FRONT 3d JWM.skp (349.7 KB)