In present tense negative sentences that contain 'नहीं', using है/हो/हैं/हूँ is optional. It is perfectly fine if you include it but it is often omitted.

The only exception is if the sentence does not contain any other verb in which case है becomes mandatory. Eg: मैं लम्बा नहीं हूँ | (I'm not tall) where हूँ cannot be omitted