Cost element error on internal activity

Hi all;

I am currently having a problem in that when I enter a value in the Mat. Planning field on the Assignment tab of an internal activity I get the following error: " Cost element 400000 does not exist in CO area XXXX"

Where can I change the assignment from cost element 400000 to another that is within the CO area?

The obvious answer would be to create the required cost element but I am trying to avoid this.

Regards;

Werner