3357826 - Error OH021 happens when adding lower level into cost element group

You want to add a cost element group to lower level of another cost element group with app Edit Cost Element Groups.
You get error:

Inserting of the object (Cost element group: DF) is not possible
Message no. OH021

SAP S/4HANA Cloud

As explained in the long text of the error message:
Inserting object (Cost element group: AA) is not possible. The object contains values/intervals that would lead to overlaps with already existing values/intervals.

It means, if you want to attach lower level cost element group, you need to mak sure there is no duplication assignment of cost elements.

To verify the same, you can use the app Display Cost Element Groups.

Menu - Extras - Check and Help Functions - Compare Two Groups
Display matches

There you will find the duplicated cost elements assigned to both groups.

OH 021, insert, cost element group, KAH3, KAH2. , KBA , CO-OM-2CL , Overhead Cost Controlling (Public Cloud) , Problem