Sometimes you might not be able to sync Core data, say projects or invoices, with QuickBooks Online. This can happen when sending over data to QuickBooks Online or getting data from there into Core.
To ensure accuracy and completeness of data synchronization, Core uses some synchronization rules. Some of these rules might not be relevant for the first-time data synchronization between Core and QuickBooks Online, but they are applicable in future. Also, there can be some limitations posed by the accounting software that does not allow sending or getting specific type of data.
Make sure the sync settings (Send/Get/Accounts) are set correctly to avoid any complications in the process of data integration. To learn more on syncing rules, check out the Synchronization Rules.