sub contracting process

Hi all

We purchsed material from vendor A & send these material to vendor B directly for Job work

In PO we tick SC vendor & give vendor B.

Then made GR & MMBE report shows stock provide to vendor B.After that we wnat to make subcontracting challan for vendor B with reference to GR number.

But system not allow to make sub contracing challan.

showing follwoing "Document already exists for material document 5000086201 2008

Message no. 8I261 "

so how to make or find document exsit>