You have to explain in detail exactly what you're trying to do before we can be of any help at all, I'm afraid. So far you haven't told us anything, except that you are trying to do something that technically you can't do - you can only export audio as a file, not to one.
i wanted to export as an mp3 file
From Multitrack or Waveform view? And why do you want to export? If you are in Waveform view, then you should be using Save As. Like I said, more details required.
I mixed two songs together and wanted to extract the end result from multitrack view into an mp3 file
Export>Multitrack Mixdown>Entire Session and then use the Browse and Change buttons to alter the settings:
Thanks for your help