"Mijn broer is niet slim, maar hij heeft een goede intuïtie."
Translation:My brother is not smart, but he has good intuition.
I agree. It should be 'good intuition'. I suppose you could use the article in a sentence like "I have an intuition about that" but even then you wouldn't say "a good intution". I would prefer 'clever' too as I only use 'smart' about appearance (UK English).