A possible rewrite: 오늘은 레인 리포트 카페에 갔어요. 밖에 비가 오는 카페예요. 그런데 제가 밖에 나오자마자 비가 그쳐서 실망했어요. '-거든요' explains something the other person might not have fully understood or might be interested to know more about. Having visited a certain cafe doesn't sound like it needs further explaining, so I think simple 카페예요 is better. 나가자마자 => 나오자마자 (since you're not in the cafe anymore now. You say 나가다 when you're in the cafe). 오기 멈췄다니 => 그쳤다니. 그쳤다니 안 돼요 sounds unclear. If you mean you don't want it to happen, you could say 비가 그치면 안 돼요. 안 돼요 is usually said about what is about to happen (or when it is happening as a reaction), not what happened in the past, so phrases -다니 doesn't work well with it. If you mean it happened and you were disappointed, 그쳐서 실망했어요 or 그쳐버려서 분위기가 깨졌어요 (it shattered the mood). However, it is possible to say something like 비가 그치다니 ... (앗) 안 돼요!, as if you're in the middle of experiencing it and screaming in reaction (this is very informal though).