Billing block appears as pricing incomplete in sales orders

In an inter company sales scenario, the inter company sales order is getting blocked from being posted into SAP because of "Prices Incomplete" billing block in the SO header.

The details for the SO come from a third party system in the form of idocs. the sales order doesnot get posted in SAP because of the billing block.

Can you suggest the reasons for which the billing block is appearing?

The SO type is not customised to have a billing block too.

Thanks

Sowbhagya