Diagnosis
You want to assign a planning group to a vendor. However, this planninggroup is already in use in customer master records. If you use it invendors master records too, a complete distinction between vendors andcustomers 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 customer master records or use another group for the vendormaster records.