You can save Sales Invoices. Create the invoice object with SalesInvoiceFactory.Create() and assign values. Then use the objects .Save(). I have tried recreating the error by manipulating the company access. I will also check single user license and permission on the directory where the company is located. Will take a while to either get other users in or switch the license to test.

I need to save Purchase Invoices.  I do understand how to save Sales Invoices.  That works fine.  Thank you for your comments here.  Much appreciated.