SAP Message BUAG006 - Same numbers cannot be ensured (contract account &1)

Diagnosis
If you replicate a business agreement in a contract accoount, thisshould be created with the same number. However, for this number, thereis no external number range for the number range objectFKK_KONTO thatis assigned to a contract account type (table TFK002A).The numberequivalency is required for certain components, for example ISU/CSS.