There are three reasons for an “Audience Not Exportable” message or an inactive “Enable Export” button.
"Exceeds 5 audience maximum" - Unless you’re subscribed to Enterprise Remarketing, you cannot export more than 5 audiences per organization.
"Exceeds 20 audience maximum" - Regardless of subscription status, there is a total maximum of 20 audiences per organization.
"Contains Facebook and/or Twitter acquisition data" - Due to their data retention policies you cannot include a Facebook or Twitter acquisition campaign as part of the definition of the audience that you're attempting to export. Facebook and Twitter limit the extent to which you can associate their acquisition campaign data with individual devices. To remediate this issue you should remove Facebook and/or Twitter from your audience criteria, and then re-enable the export.