I thought that (already ) is right but i just want to know why the word is not right? Would you please tell me the reason?
The problem is that early can not be placed in that specific position in the sentence. If you chose to use early, it would need to be placed at the end of the sentence.
10 members of the sales staff have exceeded their quotas early.
(Side note: we don't generally use digits to start a sentence, unless it's unavoidable. Here, we should just opt for spelling the word "Ten")
There is no concrete rule to decide this, early can be an adverb, and generally adverbs can be adjacent to the verbs they modify, but this one can't. Here is an interesting summary of types of adverbs and their possible placements.