Hierarchy is active - you are able to hang article on end nodes only

What causes this issue?

You are trying to assign articles in an active hierarchy to a node that
is not an end node. This is not permitted.

System Response

The system does not allow you to do this.

How to fix this error?

Hang the articles onto an end node.

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.