Hello S K,
For making a proforma invoice, you will need to have the sold-to and payer partner functions. This is the same problem I had faced when my business user wanted to create a proforma invoice against a subcontract delivery (LB). The sold-to and payer were not automatically determined when the LB delivery was created and as soon as I made them mandatory in the relevant partner determination procedure, I got the expected result. Please look at SAP note 672142 for info about this topic.
Best Regards,
Balaji