What, however, made us very frustrated was that there was no opportunity for us to take an exam at the end of this month.

frustrated 是形容词，来自动词 frustrate（使沮丧、挫败），表示"感到沮丧的、受挫的"。这个词描述的是当事情不如预期或遇到阻碍时产生的负面情绪状态。