Hi Pradeep,
Once you open your BP in GUI (Tcode BP) Select BP Role Sold to party, Click on Sales Area. There you'll find the the option of Choose Sales Area.
If there you dont see the sales Area then you need to check the Org Structure, Check all the details are given (Tupel, etc).
We have two options depends on Category Type, if you are choosing (eg MAT_DIEN) then you can see tab for Sales and Distribution there you can give the Sales Org.