This can occur if we have not selected the correct export type. Instead of adding an Export Definition, you may have chosen Ad-hoc query. 
  1. From Administration, Click Export
  2. Select your export process, Click Edit Process in the task bar on the left
  3. In the edit process screen, Change Export type to Export Definition
User-added image
  1. Search and select your selection
  2. Search and select your export definition
  3. Save