QuickBooks Mac uses Apple’s Sync Service APIs for Contact Sync. Unfortunately with Mavericks Apple removed the Sync Service APIs.
Resolution for 'Apple Sync Service issue: 'Contact sync settings is grayed out?' available: Yes.
The Apple Sync Service issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the Apple Sync Service issue yourself by looking for a resolution described in the article. If it is a complex Apple Sync Service issue or you are unable to solve the issue, you may contact us by clicking here or by using other Apple Sync Service support options.
For more information about resolving issues related to Apple Sync Service in Quickbooks/QBO or to get support for the issue 'Contact sync settings is grayed out?', click the link below: