they didn't have any problem _____the post office. 空格处为什么要填finding而不是to find?

固定搭配
have problem(trouble) in doing sth.
in可是省略的
或者你可以这么想 后者表示目的 前者表示结果
他们已经找到邮局了 显然是结果

have difficult/problem/trouble (in) doing sth
在某方面有困难
固定短语

finding
have problems(in) doing 是固定词组
类似的词组还有
have touble/difficulties/problems (in) doing
have fun doing
have a hard/difficult/good time (in)doing
动词不定式表目的多一点
“没有困难为了找到邮局”
感觉他们中间用表目的的不太合适
还是把它当做固定词组记吧 我就是这样记得O(∩_∩)O哈！

have problem(in)doing sth