Why I can't cut there with the knife tool, I want to make a quad out of this n-gons
You can Cut Trough by pressing Z while knifing
Allow the Cut tool to cut through to obscured faces, instead of only the visible ones.
or using Knife selection Shift-K (by default)
Activates the knife with another set of options so only selected faces are cut and Cut through is on by default.
The issue seems to be because the face is obstructed by other faces.
Try to hide all other faces by pressing Shift+H,
This will hide all faces, now you should be able to cut it easily, I suggest you to turn on vertex select mode to do so as you will see the vertecies more easily.
To unhide it all you press alt+H