φτιάξει Is a much better verb duos translation is more like the child wants to do a sandwich.
We have φτιάξει as a choice but κάνει seems more widely used from my experience.
Why is it wrong to say ένα σάντουιτς?
"ένα σάντουιτς" is a correct and accepted translation if you had a problem please give us more information and a screenshot. We have no way of seeing the answers you give.
Why is σάντουϊτς considered a (small) mistake? Are the umlaut not allowed in the monotonic system?
σάντουιτς.. is a two syllable word adding the diaeresis would turn it into a three syllable word. Borrowed words often have odd spelling until they are incorporated into the language thoroughly.