Diagnosis
Settlements were already performed for the settlement unit, or thesettlement unit is being settled at the moment. The settlement unit isnot allowed to be changed in periods already settled or forperiods currently being settled.

Procedure
You might need to make changes to periods that are not settled, forinstance, because different apportionment rules should be used. In thatcase, you can add a new settlement period and create the apportionmentrules for that settlement period.
If you need to make changes for periods already settled or for periodscurrently being settled, then you have to reverse the given settlementfirst.