This sign, which incidentally is perfectly grammatical, seems to suggest that only restaurant food that is fortunate is allowed in.
If, however, you were physically there and knew it was the entrance to Fortunate Restaurant, then you would understand it to mean only food ordered from the restaurant may be consumed there.
Syntactically, the noun phrases are structured differently. The intended interpretation has food as head noun and Fortunate Restaurant as noun premodifier. By contrast, the unintended meaning has food as head noun also, but with two premodifiers: the adjective fortunate and the noun restaurant.