The package cannot be deleted because it still contains objects

What causes this issue?

The package you entered is not empty; it still contains development
objects.
This message can be displayed if the package only appears to be empty.
Some object types (classifications or OTR texts) can still be assigned
to the package, although they are not displayed in the tree display in
the Object Navigator (SE80).
The message can also be issued if all the development objects of the
respective package have been deleted, but these deletions have not yet
been transported.

System Response

The system cannot delete the package.

How to fix this error?

First delete the content of the package.
For more information on the procedure for deleting packages that appear
to be empty,. refer to SAP Note number 1156585.
Release the respective transport request (including all the tasks
contained therein).

Error message extract from SAP system. Copyright SAP SE.