As your book uses the terms:
- Elements: Single atoms (e.g. O).
- Compound: Consists of only one type of molecule, where that molecule consists of more than one element (e.g. H2O).
- Mixture: Consists of more than one type of molecule (e.g. Na+ + Cl- + H2O, which is saltwater, or for example the air you breathe, which is mostly N2 with some O2 floating around and a bunch of other stuff).
I'm not entirely sure where your book classifies materials that consist of a single type of molecule where that molecule consists of only one element (e.g. O2) but whatever, you should be able to fit that in somewhere. It probably includes those as "elements".
To use a loose macro analogy (don't deconstruct this too deeply or you will soon run into trouble), let's say "blue paint" and "yellow paint" are elements. When combined they form the compound "green paint". On the other hand say "blue marbles" and "yellow marbles" are elements. When combined, they don't form "green marbles", they just form a mixture of blue and yellow marbles.
Note also that samples of your pile of blue and yellow marbles may vary: If you pick up a handful of them you very well could end up with slightly different proportions each time. On the other hand, every sample you take of the green paint will be identical green paint.