Download CSV functionality is unavailable because this campaign doesn't include a merge tag.
Why are merge tags required?
Merge tags are used to insert unique user data from your mailing list into emails, such as a recipient's email address or first name.
With a unique merge tag (like an email address), we are able to tie data we've gathered to individual users and make that data available to you via
CSV download.
Unfortunately, once a campaign has been activated it is not possible to add a merge tag.
If you would like to learn more about merge tags, please visit our Help Center.