RE: The record in table Purch. Inv. Header already exists. Identification fields and values: No.='PREVIEW'
                     
                    
                      
                        Hi, do you have any customizations on posting?
I've had this happen once before, after using one of the posting events.
If there is customization, I think you can copy a Sandbox environment and debug in it.
If this is a standard issue, I suggest you submit a Service Request to Microsoft.
Hope this info will help.
Thanks.
ZHU