"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.
Am I mistaken in thinking 'usted' could be placed either after 'Señor' or at the end?
Why on earth is 'mensaje' wrong here. A note is a message so this should be correct
I will give you that 'una nota' can be one way to deliver or pass on 'un mensaje'. However, I feel I must point out that while a note by it's definition must be a message, a message doesn't necessarily have to be a note.
The choice of using the word 'note' over 'message' in English here, infers that there's some unspoken need or desire to be specific about how the message was delivered.
At least this is my reasoning of it. I hope this helps!