How to fix hollow walls in sketchup

How can I fix the hollow part in these walls please? Beginner here & not sure why they ended up like this? Maybe to do with angled ceiling & different wall heights?

Hard to tell from your screen shot but to get a face you be a connected loop of coplanar edges.

From just a screenshot it’s hard to be sure what you mean by “hollow”. SketchUp models surfaces; there is nothing inside a “solid”.

This is how SketchUp works, since it is a surface modelling software.

Walls are hollow since a wall (or floor or roof) that has some thickness is modelled using two faces, One for the inside of the wall and one for the outside of the wall.

Each of these faces has no real thickness but has two sides. You must respect the proper orientation of the sides of a face if you are to use a rendering software.

Post the skp file with which you have problems so we can help you with ideas to do the modelling that you have in mind.

Sketchup uses English but not as we know it (Jim).

You may think a cube like a die is a solid but a cardboard box isn’t. But Sketchup calls the latter a solid because it has conjoined sides without breach. You clearly have a breach in your model (for reasons unclear without seeing the file).

Similarly, you may think that joining two points by the shortest means would give you a line. But to Sketchup, it is an edge. Even when it has no surface attached to it, when it is just an “edge in waiting”.

If this sounds like wordplay, it isn’t. It is crucial understanding for using the software.