This was for trying to make a trade with more than 10 items, I assume it's supposed to say "Cannot make a trade more than 10 items". It was confusing to me at first since I didn't realize at once that I was trying to make a trade with more than 10 items.

I have gotten that message when I have forgotten to click to add anything to a trade. I think it is a clever way of telling us we messed up

I went to make a trade to test and it is still happening. I can make a screenshot if you need.