Planning object structure '&1' could not be deleted

What causes this issue?

Planning object structure '&V1&' could not be deleted because it is
still used in a consumption group for the associated planning area '&V2&
'.

System Response

The deletion process has been terminated.

How to fix this error?

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.