Precise English speakers would say, "The tree has only one leaf" to express the intended meaning. This makes "only" modify "one." This is logical but doesn't seem to reflect the Vietnamese grammar. In conversation, many (if not most) English speakers actually say, "... only has... " which isn't literally true because the tree obviously has roots, branches, etc., in addition to that leaf. I am, of course, nitpicking as far as conversational English is conncerned but, if we are talking about what is correct or what should be accepted, then, I think, logic and grammar have a place in the discussion.
I wrote "The tree has only one leaf," not only because it is the correct way to express what I believe is the meaning the Vietnamese sentence is after, but also because I was curious if DL would accept this translation. Sure enough, it was not accepted. Once again, getting through one of these lessons requires spitting out poor quality or incorrect English (the tree only has one leaf), not because I don't get the Vietnamese to translate it correctly, but because the writers of these lessons are not familiar enough with English to accept correct translations.