R2900

An extension relationship must reference at least one Extension Point.

An Extension Point is a feature of a Use Case that identifies a point where the behaviour of a Use Case can be augmented with elements of another (extending) Use Case. Therefore, an extension relationship should reference an Extension Point of the Use Case it extends.

Add an Extension Point to the Use Case you want to extend if none already exist, and/or reference that Extension Point in the extension relationship properties.