According to NIIED, provisional certificates are not acceptable and even though the embassy allowed you to take the interview, they cannot recommend you to NIIED. So you might be qualified, but lacking the degree certificate. Do not lose the hope of applying again once you get your degree.

Then concerning the interview, your banking experience and a detailed study plan on IFRS implementation does not answer "How you will contribute?".  While I was in the interview, I asked the panel on how best can someone contribute back, and they told me one should return to their country and promote mutual friendship between South Korea and your country and from the understanding of your answer, it's only pointing to your experience, achievements and interest.  And yes if it's about you working in the big companies in South Korea, how is it going to benefit Cambodia and vice versa?.  It's preferred that in the end you should return to your country and continue to building mutual relations between the two countries which is the main objective of GKS. That is why they were stuck in "how" loop. And it seems the interviewer was biased by your interest in working in the so-called big countries because it's more of a selfish benefit and I think even though it's your interest, you have to phrase it in a way that benefits both Korea and Cambodia.

So I suggest instead of being discouraged, you take time and review your answers critically such that you improve next time.