Third party scenario

Hi Friends,

I am learning SAP SD , Where I am doing third party process , i got two issues

1. when i created a material with BANS as item category group. the system through a message saying that it is internally procured.

2. when i am creating migo with reference to purchase order , the system throughs error message "the purchase order has no items"

request you to all to guide accordingly.

Thanks

Ken