No CO class entered, although the CO part has been maintained

What causes this issue?

You have overwritten the planning parameter class with a new value.
However, this new class does not allow you to use the parameters in
cost accounting. You maintained the special CO part previously. This
means that you cannot change the planning parameters to this type of
class.

System Response

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

How to fix this error?

Enter a parameter class which allows you to use the parameters in cost
accounting.

Error message extract from SAP system. Copyright SAP SE.