Subcontracting Process

Hello SAP,

I have a issue at the time of subcontracting process..

1. I have created a P.O with an Item category L to a vendor X. for processing semifinished material. For this I exploded BOM

2. Now i have to provide the material i.e. raw material to the vendor X for processing semi finished product. By using T code MB1B, MVT 541, with respect to P.O. At this time system giving error message: ENTER A VENDOR FOR SPECIAL STOCK O.

Please suggest where to make the backend settings for the above error message.

Regards,

Karan