Diagnosis
You want to delete the reference type &V1&. However, this is notpossible because the reference type is still used in the document typeV2 for the document category v3.

Procedure
If you want to delete the reference type &V1&, you have to first deletethe reference to the reference type for the document type &V2& for thedocument category &V3& in the step Definedocument types.