"You do not have to be so formal."
Translation:No tienes que ser tan formal.
As a Spanish learner I can imagine times when estar is appropriate for this translation e.g. You don’t have to be so formal (with my friends only with my co-workers.) I’m not referring to how one should dress, but rather how one would act. Would a native Spanish speaker never use estar? It wasn’t accepted by Duo in my response: No tienes que estar tan formal.