请问这是个病句吗？

that后接完整的句子

mean to do sth.有意做某事。
that后面得接从句，也就是一个完整的句子。所以 不能这样接。
i think he means to look for trouble.

There's a man at the reception desk who seems very angry and i think he means (to look) for trouble.

mean to do sth.打算做某事
mean doing sth. 意味着做谋事
应该吧 that 删掉吧