Delibery output to sold to customer

Dear Friends

I need to send the delivery output to sold to customer. However Sold to field is not there in field calaog. I also pulled in the fields KUNNR or KUNAG from available flields list, but at the time of access sequence creation, I get error - The field assignment has not been made and field is shown in red in the fields tab..

How can I fix this.

Thanks and Regards,

Neeraj Srivastava