You can eat free in the restaurant ___ you like.为什么只能填whenever而不能用whatever呢？

You can eat free in the restaurant whenever you like.
意为：无论何时你都可以在餐厅里免费吃。
分析：whenever you like作时间状语从句。

You can eat whatever you like in the restaurant for free.
意为：你可以在餐厅里免费吃任何东西。
分析：whatever you like置于eat之后作为其宾语，在此处是一个宾语从句。

whatever指的是任何东西

餐馆里边没有任何东西吧？！

You can eat free in the restaurant whenever you like

You can eat whatever you like in this restaurant for free

应该都行，一个是指随时过来都行，一个是指吃什么都行。
感觉whatever还要好点。

无论何时