"I do not cook it."
Translation:Ik kook het niet.
No because 'geen' is for where you'd negate 'een', there's no indefinite article here. You are saying you are not cooking it, meaning you are not cooking something on its own. You cannot say 'I am not cooking a it' (as that doesn't even mean anything), it has to be niet. You can use geen to say: ik kook geen kip or ik kook geen appels etc. But for this sentence, it is: ik kook het niet.