Exporting to MP3 not working

 I'm a novice to Audition but have put out a few podcasts and not had this trouble before. I'm using multitrack and edited/recorded everything onto it ready to export. Only the option to do so is greyed out on the menu. I've checked the dropdown boxes and all are set to 'Master' so that doesn't seem to be the issue. Not sure what to try now really. Any help would be much appreciated.   

Thank you!