Settlement fields are maintained in the order -> display only

What causes this issue?

You have maintained the fields 'Settlement Cost Element' and either
'Cost Center' or 'G/L Account' in the 'Settlement to One Receiver' group
box in your order master.

System Response

The system automatically created a settlement rule specifying 100%
settlement to the appropriate receiver.
Once this is done, however, you can only display the settlement rule,
you can no longer maintain it manually.

How to fix this error?

If you want to create a settlement rule manually, you first have to
delete the contents of the 'Settlement Cost Element', 'Cost Center, and
'G/L Account' fields in the 'Settlement to One Receiver' group box for
the internal order master data.

Error message extract from SAP system. Copyright SAP SE.