Diagnosis
You want to assign a planning group to a customer. However, thisplanning group is already in use in vendor master records. If you useit in customer master records too, a complete distinction betweenvendors and customers will not be possible.

Procedure
If you do not need to distinguish between customers and vendor data,you can go ahead and assign the group.
If such a distinction is necessary, you must either remove the groupfrom the vendor master records or use another group for the customermaster records.