Who eats just one strawberry? This is a weird sentence, at least in English. "A vegetarian does not eat beef but strawberries" would sound more natural (although it's still making an illogical false dichotomy of beef or strawberries being the only options for food).
When you hover over "fragon," duolingo tells you the translation is either "a strawberry" or simply "strawberry." If that is true, then the translation "A vegetarian does not eat beef, but strawberry" should be allowed.
This is a weird sentence to say in English. Vegetarians would more likely be used in this context. And the distinction between beef and strawberries is strange too. Shouldn't it rather be meat and vegetables. And "a" strawberry? Just strange all over.