Invalid Reference Id : Items element id not found
This error is typically caused by a user selecting Categories rather than Product Codes when line items are added to an invoice.
To resolve this, it would be advisable to first navigate to your company's Integrate page, and select the option highlighted in the image below before saving the changes. This should prevent the issue going forward.
To clear the error from your invoice(s), you will need to do the following:
Unpublish the invoice
Change the categories to Products/Services and then revert back to Categories
Publish the invoice again