Learn Turkish: Misafirler için içinde alerjen olmayan bir menü hazırladılar. - They prepared a menu that does not have allergens in it for the guests.

This is an existential relative clause, which is the standard Turkish way of saying a noun "has" or "doesn't have" something. Instead of a verb like "to have", Turkish uses a location (içinde, meaning "in its inside") plus the item (alerjen) plus olan (which is) or olmayan (which is not). Literally, it translates to "a menu which allergens do not exist inside of".

Why do we use içinde here instead of just attaching the locative suffix to the menu, like menüde?

Because menü is the main noun being described at the end of the phrase. You can't put the noun inside its own descriptive clause (e.g., you wouldn't say menüde alerjen olmayan bir menü). Instead, we use içinde ("in it") as a placeholder to refer back to the menu we are describing.

Why is it alerjen and not the plural alerjenler if the English translation says "allergens"?

When talking about the general presence or absence of a type of thing (especially with var/yok or olan/olmayan structures), Turkish usually keeps the noun singular. Alerjen olmayan functions a bit like the English phrase "allergen-free", treating the allergen as a general category rather than counting individual items.

Why is bir placed right before menü instead of before the whole phrase?