“Cannot be happening” expresses shock and surprise about something that actually is happening in real life as you speak, since the fact that it's happening proves your sentence can't apply literally. Does the Indonesian sentence mean that? I feel without a progressive aspect in your language to express that nuance, you'd need to say something like “Saya tidak pernah berpikir bahwa peristiwa begini mungkin terjadi.” Should we translate this to “This event can't happen” instead?
"tidak mungkin" = not possible, impossible.
Literally: "This event is impossible to happen/occur."
The ID sentence can mean that you're surprised that it actually happened.
It can also mean that you expect that the event will not happen, because you think it's impossible.