"You are the boy I like most."
Translation:Du er gutten jeg liker mest.
I'm not an expert by any means but I believe the reason that isn't accepted is because 'den gutten' translates to 'that boy' in English rather than the definite article it's asking for ('the boy'). 'You are THAT boy I like the most.' also sounds odd to my native English ears as a standalone sentence - which I assume this sentence is meant to be perceived as. Unless it's being used to follow up a previous sentence where 'that boy' was used as a noun (such as 'That boy I mentioned? You are that boy I like the most') it doesn't sound right to me.