"I know you."
Translation:Ich kenne Sie.
If anybody is wondering what is the difference between "kennen" and "wissen" since both can be translated as "to know":
"kennen" means "to know, be familiar with" and "wissen" means "to know a fact, know when/how."
German-speakers always know (wissen) when to use which one. If they are talking about knowing a person or being familar with something, they will use kennen. If they are talking about knowing a fact or knowing when something will happen, they'll use wissen.
"Ihnen" is wrong. You could say "Ich kenne dich" (informal singular) or "Ich kenne euch" (plural).
Why is Ihnen wrong? I generally am confused as to why it was rejected, especially as you can say dich and euch.
'kennen' takes an accusative object, not a dative one. 'Sie/dich/euch' are the accusative forms of 'Sie/du/ihr' respectively, 'Ihnen/dir/euch' are the dative forms.