for those wondering why it is à voir here is the explanation taken from Michel Thomas's French course::
In English have to does not always mean must. For example, What do you have to drink?
Any form of avoir followed by the infinitive of a verb has the structure:
conjugation of avoir + à + infinitive of verb
Qu’est-ce que vous avez à manger? - What do you have to eat?
Qu’est-ce qu’il y a à boire ? - What is there to drink?
je n‘ai rien à faire. - I have nothing to do
Je n'ai rien à couper le papier - I have nothing to cut the paper
qu'est-ce que vous avez à balayer le plancher ? - what do you have to sweep the floor
Je n'ai rien à ajouter. - I do not have anything to add.
They're being pretty pedantic, I agree, but I think technically "not anything" is more "aucun," while "rien" is nothing. The two sentences have the same meaning in this context, but I think that Duolingo is enforcing the difference here, as that is not always the case.
Il n'y aura aucune chose à voir = There will not be anything to see
Il n'y aura rien à voir = There will be nothing to see