User is unable to send invoices from Core to QuickBooks Online and gets an error.
The error might be caused if the invoices are billed with a negative value. QuickBooks does not support syncing of negative, draft or $0 value invoices. Another reason can be that the prerequisite data is not being synced first. So prior to syncing invoices, you need to sync the related data such as employees, projects, clients, etc.
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 are applicable in future. Please check the Core Help for all the sync rules.