UK Don't worry, this error is only because the number of pages in the file you have uploaded doesn't match the specification you have selected.
Be sure to upload the correct amount of files/pages, so for example if you've selected 'both sides' sides for print, you'll need to either upload a two page multipage PDF or two files.
Top tip: even if it's the same artwork file for both sides, please upload two - otherwise you'll risk not having the print you're expecting!
You can check your specification by clicking here at the bottom left of the page