Translation:The aunt thought that the boy did not like that game.
There needs to be more consistency in the pattern of translation: ' que ' after verbs of saying and thinking etc. is very often omitted in the given translations, which is commonly done in English. However, occasionally it is translated, as it is here. The student has no way of knowing which way to jump, and loses hearts for no good reason! Ouch!
My first instinct is to write "the aunt."
If he is the the boy, then I think it'd be better to say he instead of the boy. His aunt thought that he had not liked the game might mean what Duolingo's sentence means in some cases, but I would guess it is not accepted.
This sentence could mean that, or it could be a different aunt thinking about a different boy.
When "pensar" is used in the past, it often points to a counterfactual statement. What the aunt thought was true at the time turned out to be wrong. The boy did like the game.
Another point here. This translation from Portuguese to English reflects Portuguese syntax, not English. (We think something or we don't think something...)
The aunt didn't think that the boy (had) liked the game.