relation 和 relationship 的区别是什么?比如我现在想说 develop close relation(ship) to client representatives.这句话用relation 还是relationship呢?

多了一个ship

relation：“关系，关联”
relationship：关系，这个更侧重具体的关系。

As to your sentence,you can use either "relationship" or "relations."
Please note that you should use "relations" instead of "relation," since while the former means "the way in which two or more people,countries,or organizations feel about and behave towards each other," the latter actually means "the way in which two or more concepts,objects,or people are connected; a thing's effect on or relevance to another."
As to your question about the difference between these two words,I think my foregoing explanation has already partially answered your question.Relation and relationship,when they mean "the way in which two or more concepts,objects,or people are connected,or the state of being connected," they are practically the same.However,you should use either "relations" or "relationship," but not "relation," while referring to "the way in which two or more people or organizations regard and behave towards each other."
Hope that my answers help.