Why are my walls hollow? and I can't push through them?

I’m having trouble with my model. I can’t push walls and my walls are hollow if I look under my model. Is there a way to fix this?

Please don’t double post.
Why have you said in one thread that , yes, you need to watch some more tutorials and then have immediately posted the question again.
You were give a link to the campus in the other thread and if you had spent some time there your questions would probably already be answered.

The holes in your model are due to the way sketchup works, if you pushpull a single face it will make an enclosed shape, if you pushpull a face that is part of another face it will make an indented shape, if you tap ctrl while doing it it will replace the missing face. But if you had spent the time to learn how the tools work, rather than just say that you ‘know how the tools work’, you would have learnt to use groups and components to stop you getting hollow walls etc
As for the hole not pushing through, as mentioned in the other thread, your walls are not straight which will make creating holes troublesome.
Turning Off Length snapping in Window/Model Info/Units might help with your inaccuracy.

Personally, my recommendation would be to go through the courses at the campus and then start your model again once you have a better understanding of the tools. This will save you much frustration.