"Don't look out the window all the time."
Translation:Nedívej se stále z okna.
Stále nedívejte z okna - should not be accepted as a typo and you are correct, it is. I cannot figure out why. It does need the reflexive SE. Stále SE nedívejte z okna.
On the other hand "Stále necivějte z okna" is correct though rare. CIVĚT is to stare, is not reflexive and is used a lot but has two forms of command form. CIV(TE) or CIVĚJ(TE) (in more common negation NECIV(TE) and NECIVĚJ(TE)). And while NECIV is very common, I had to look up NECIVĚJ to make sure it is a proper form. It is. Why Duo picked that one to offer to you as one of the many possible alternatives is a mystery.