You have to be perfect.
"Il doit être parfait." and "Il faut être parfait." Duolingo only takes 'faut' as the correct answer, but don't they mean the same thing?
They're slightly different in this case. The first one would mean either he has to be perfect or it has to be perfect, whereas the "il faut" sentence is more one has to be perfect, you have to be perfect, or it is necessary to be perfect. In other words, you'd never use the first sentence as a translate of "You have to be perfect."
You probably mean "Tu dois/vous devez ..." vs "Il faut ...".
The difference is that the first explicitely refers to you, while the second is a generic reference, and could just as well translate into: "We have to...", "One has to..." or "...has to be done."