Going back to square one, as far as I can see your notebook is essentially a list of topics, organized in cells of various styles. But, for example, if I look at topic "Foundations", the big cell associated with it doesn't have a format style, like Chapter, Subchapter, Section, etc. Select the big bracket, second from right, which starts with Foundations. Then click Format|Style and you'll see that there is no style checked in the list. This I don't understand.
Now, when you say in your last post that you just had to add Fermat's Little Theorem and Euler's Generalization, I don't know what you mean: the two titles are there, with nothing inside them, as the rest of the cells. Do you mean you were about to add content to those cells?