Question

Why did Einstien not get Noble prize for his energy mass relation E=mc^2?Is there any flaw in it?

Answer 1

The Nobel Prize generally limits itself to one prize per person in any given field.

Arguably, the photoelectric effect (for which Einstein was awarded the prize) was even more important than that, since it was a founding discovery for quantum physics. 

Einstein showed that light was itself quantized. Electrons were also quantized, as De Broglie said. That means, basis for quantum physics was formed.

But a point to be noted, Einstein was award Nobel prize for his services to theoretical physics, and especially for his discovery of the law of the photoelectric effect. 

Which means, he was awarded it because of his numerous achievements in physics including The photoelectric effect.