SAP Message R1781 - Partner &1 already has the maximum number of relationships possible

Diagnosis
Included in the definition of a BP relationship category is the maximumnumber of BP relationships of the same category and samedifferentiation in which a partner may appear as business partner 1 or2 at the same time.
You tried to add another BP relationship to this partner. There is anevent in the validity period of the relationship to be created in whichthere are already so many BP relationships for this partner that themaximum number is used up.

System Response
Error message
You cannot create a BP relationship with this validity period.