You are seeing the below error message when attempting to publish an invoice.
Relation specified incorrectly
SortMyBooks integrated companies.
This error can occur if the company is not set up as a supplier/sub-contractor (Purchases) or as a customer/principle contractor (Sales).
SortMyBooks can only accept published purchase invoices to a company set up as either a Supplier or Sub-Contractor. To change the company setting in SortMyBooks, navigate to Supplier Settings > Financial > then change the Payee ID to Supplier or Sub-Contractor.
If you are publishing sales invoices to a customer account, the company needs to be set up as a Customer or Principal Contractor. As with a supplier, if Other is selected then SortMyBooks will not accept sales invoices.