What does Flattering eye of sleep mean in romeo in Juliet?

You have misquoted. The phrase, from Act V Scene 1 of Romeo and Juliet is: "If I may trust the flattering truth of sleep, My dreams presage some joyful news at hand." And what Romeo is saying is that he has had a pleasant dream and he feels good about it. The dream tells him there is some joyful news at hand. But he is not sure he should trust it, because the truths of dreams can flatter, can tell you what you want to hear. The "flattering truth of sleep" means dreams that tell you what you want to hear.