3576921 - Cannot Post B2B Supplier Invoice due to Error Message

Unable to post B2B supplier invoice due to errors. 
Errors: 
Enter a product ID, product category, or G/L account for item 5.1; see Cost Distribution Details.
Posting is not possible. Invoicing unit has no employee assigned. Contact a user administrator to assign the default employee responsible.
Tax code missing

SAP Business ByDesign

1. Go to Supplier Invoicing work center
2. Go to Invoices and Credit Memos view
3. Select the relevant Supplier Invoice and select Edit
4. Click on Check button & error appears

In the document flow of supplier invoice there is a customer invoice linked to this invoice. However customer invoice ID does not exist and the system cannot find a basis for the document. 

In case of B2B scenario, always customer invoice ID represents external document ID of the B2B supplier invoice. Since the customer invoice is not present in the system, user can choose to re-issue the supplier invoice or user can manually create and post the supplier invoice.

Enter a product ID, product category, or G/L account for item 5.1; see Cost Distribution Details; CL_BDF_PI_ERRORS_AND_TASKS_SELCP;    CL_BDF_PI_ERRORS_AND_TASKS_SELCM005  , KBA , SRD-SRM-SI , Supplier Invoicing , Problem