SAP Message 60B000 - You cannot change the option rate for month &1 year &2

Diagnosis
The option rate for company code <(>&<)> business entity <(>&<)>property <(>&<)> month <(>&<)> year <(>&<)> has already been used foraccount assignment; you may not make any changes to it.

System Response
The system will not change the option rate.