Hello Panos, and welcome to the forums - glad you could join us!
Congratulations on the Jupiter 21M. A word or two of advice: my own 21M has a rather long aperture pin that fouled the two SDM contacts just inside the camera's lens mount when screwing the lens into position - the result being that it deeply scratched those contacts on my old K-5 and bent the aperture pin on the lens. I don't know if Nikon has anything like those contacts, but either way I recommend fitting the M42 adapter to the lens first, then mounting that to the camera. Also, some M42 adapters aren't very precisely machined, and can produce tiny bits of debris when a lens is screwed into position - so again I prefer to fit the lens to the adapter first, then use a blower to ensure the rear of the lens and mount is debris-free, before mounting.
Let us know how you get on