Object R3TR &1 &2 (&3 &4) is still assigned to package; see long text

What causes this issue?

R3TR &V1& &V2& belongs to the development object &V3& &V4& and is still
assigned to the package to be deleted.
Read the corresponding section in SAP Note 1156585.

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

System Response

How to fix this error?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.