"You are not similar."
Translation:אתם לא דומים.
I might not be the best Hebrew speaker, but shouldn't "אתה לא דומה" or "את לא דומה" be correct here as well? can this even work without context?
Similar to what? If it's aimed at a single person, then it's not a full sentence, whereas when it's aimed at two people, you're commenting on their likeness to one another, so it is a full sentence.