"Hij heeft veel vragen over het menu."
Translation:He has many questions about the menu.
I think you mean "De ober en de serveerster kunnen antwoord geven op vragen over het menu" or "De ober en de serveerster kunnen vragen over het menu beantwoorden".
We say in our area "We have a lot of questions over the menu" Why is this not accepted. Over is used to provide a sense of review.
So, I cannot tell if Duo accepts "vele vragen" as a valid answer here but here is my understanding.
"veel" is not an adjective but a determiner, therefore it inflects for indefinite/definite usage not based on the gendered/neuter distinction as adjectives do.
As a native English speaker who would use this sentence in context I feel as though "vele" is incorrect here (although it would probably be acceptable in standard Dutch - https://onzetaal.nl/taaladvies/advies/veel-vele) because I would argue/interpret it as he has many indefinite questions (veel vragen) and not a list of many specific questions (vele vragen).