Sometimes when a user updates or Activates an Adobe Target Activity, they get an "Activity Sync Error" which means the activity doesn't sync with the Target back end, so they'll never actually be served up on a client site.There isn't any particular activity setting or configuration due to which sync errors are triggered.Typically, it could be that the activity/audience is in a bad state (could be corrupted, too big, incompatible).
However, the following are common causes of synchronization errors derived from the past use-cases:
1. Exceeding target activity limitations- Each Target activity has some content limitation. Please see link
2. Audience Issue- Either there is an error in the entry audience or in one of the audiences attached to the test.
NOTE: Try clearing the entry audience and apply the "All Visitors" Audience and see if the activity works when using 'all visitors'
3. Broken Profile Scripts- Check the profile scripts associated with the activity and resolve any of the broken profile scripts
4. Lack of access to the specified report suite for Activity using Analytics as reporting source for Adobe Target
For any further help on "Activity Sync Error", you can ask the Adobe Target experts in the Adobe Target community or ask us on Twitter at @AdobeExpCare (http://twitter.com/adobeexpcare)or reach out to Adobe Client Care