Transaction OWML - unable to change Lifo / Fifo Settings

Dear Collegues,

i actually want to activate the setting LIFO valuation at valuation level "valuation area", but the options for Life / Fifo are all grey and can not be changed. Can someone give me a hint, how to get the settings modifiable? The System is ECC6.0.

Thanks a lot in advance

Best regards

Carsten Klatt