Mirror Sync Option Not Available

I just tried to select a mirror sync on a client site for dropbox using snapshot. Wordpress is up to date and Dropbox is selected as the destination.

Please advise me of how to resolve this issue.

One other thing, I reopened this topic here a yesterday and it hasn't had a reply: https://premium.wpmudev.org/forums/topic/i-just-tried-to-select-a-mirror-sync-on-a-client-site-for?replies=8#post-850658

Thank you.