"Sorry we're unable to offer you a return as your item(s) were on sale" - what does this mean?

Your customers reached out to you stating that they received this message. And you haven't excluded any items nor do you have any items on sale? 🤷🏻‍♂️
This is a default message that gets displayed if you have set up and activated an Advanced Rule. Of course, this can be modified so the message is displayed according to your return policy.
Heading over from your Dashboard -> Translations -> Customize -> Select language. Once you're there, just search for the text you want to change, modify and hit that Save button. That's it. Your work here is done.