Multitrack recording / codec error when converting

I am using Cool Edit Pro to create commericals for a radio station. When I create a multitrack recording and mix it into 1 track, then try to convert the .wav into a mp3 I receive the error:
CODEC required to compress (file name) could not be found.

I have no problem when I convert a single track recording to a mp3.

Why would there be a difference between the 2??

Is there a fix I might be able to download??

Please respond.