"Sir, did you write this note?"
Translation:Señor, ¿usted escribió esta nota?
Because if you're using Senor as a title, then you don't know the person and you need to use the formal 'usted' you, the informal tú is not appropriate here.
It's either a missing translation or because the sentence starts with Sir, they want you to use the formal you (usted) in the translation. "Escribiste" is used with tú only.