Aside from the fact that many of your independent parts have undefined sketches in them, the reason your assembly shows that it is undefined is because of the way it has been built (mated). You can't have it moveable in that fashion and expect it to be fully defined. I noticed you are using an educational version. Have you spoken with your teacher about this?
If you fix all your parts in place your assembly will become fully defined, obviously that's not what you need for this assembly. As Dave Bear mentioned, you have parts located by mates that allow movement & so the assembly will be under defined.