"Sin tomate, por favor."
Translation:Without tomato, please.
It isn't really a test of English grammar - and if you are describing tomato as a substance/uncountable, then just 'tomato' is correct (as you might for finely chopped or puréed tomatoes, in which case saying 'no tomatoes' does not makes sense).
I tend to say thanks rather than please after a request like the ones in this lesson. Have messed up a few times on these.