When you open a 2014 document in 2015 it will open as a [Converted] document, and exporting cannot be done until you resave the document.
It's highly recommended that you resave your document under a new version name so it's distinct from the 2014 version.
As with each addition to InDesign the database structure of your documents change to accomodate new features or features that are rewritten for better performance etc.
Therefore, files from 2015 are not compatible with 2014, therefore keeping the versions separate makes sense for everyone.
After all that - I've no clue why it's not working for you as you haven't given enough information,
What's the error message?
Have you tried any of the FAQ at the start of the forum?
It is as I said. I opened a file I created the previous day in 2014 after I updated to 2015 I get en error message saying "Background Task failed" with no error message or further explanation as to why I cannot export a PDF.
I see - sorry I think I read it wrong.
Check out the FAQ at the start of the forum - File Crashing on Output.