Reference type & cannot be deleted; It is still being used

What causes this issue?

You want to delete the reference type &V1&. However, this is not
possible because the reference type is still used in the document type
&V2& for the document category &V3&.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

If you want to delete the reference type &V1&, you have to first delete
the reference to the reference type for the document type &V2& for the
document category &V3& in the step <DS:SIMG._ISPSFM_FMU1> Define
document types</>.

Error message extract from SAP system. Copyright SAP SE.