Bob is waiting us in the classroom 哪里有错?这句中,哪里错了? 是is 还是 waiting 还是 in 还是classroom? 还有一句What are you talking? 哪里错了?

Bob is waiting us in the classroom 哪里有错?
这句中,哪里错了? 是is 还是 waiting 还是 in 还是classroom?

还有一句
What are you talking? 哪里错了?

waiting -> waiting for等待 (wait 是不及物动词,）
What are you talking about?( talk 是不及物动词,talk about '谈论',

如果你肯定有错的话那就是waiting for us,talking about 不过原句也没什么问题，最多偏口语化一点

waiting for us
wait为不及物动词后必须加介词

wait后要＋for＋某人，taking后＋about为What are you taking about。