"yok" and "değil" take some time to master. They are used interchangeably in many contexts, but not always.
yok = there isn't (something)
değil = it is not (there)
She is not home = O evde değil ~ O evde yok.
There is no bread at home = Evde ekmek yok. (Evde ekmek değil is simply WRONG)
I guess you just need to memorize as many examples as possible to get it eventually.