When it becomes sentient, then it earns the right to talk to me like human would. Until then, don't pretend to be something that you are not.
Edit: Maybe I'm weird, but I would prefer it to say something like "SCAN NOW", in a robot voice. Maybe add an "OK" if you want to soften it up a bit.
I think many 'normal' users need frequent reminders about that lower status. Many users, when confronted with a message such as "could not find file 'foo'", will feel that that is their fault.
That said, perhaps the most correct thing it could do is use visual iconography and not say anything. I'm sure the non-english speakers don't really "get" what it is saying but they have learned which steps go after each utterance.
If you were standing at a checkout and the cashier wanted you to swipe your own card, he/she might say, "scan your card please".
But, while the words read as an imperative statement or command, you'd probably notice that the intonation is one akin to a question. For instance, the voice would rise at the end, as in inquiry.
I suppose it could say, "The process will begin when you start scanning items." but it's simpler to use the honorific.
In error messages apologizing is present perhaps because the service requested could normally be performed but due to circumstances it cannot.