Price change allowed check box in Servcice PO grayed out

HI

would need your expertise to resolve my issue.It is related to Price change allowed check box in Servcice PO.It is grayed out though i have tried all the option suggested by you,But it still grayed out.

1) transaction - me22 - Price and value fields - Price chgble. at service entry - Optional

2) activity type - Which field to be maintained for service PO??

3) document type - Which field to be maintained for service PO?? THERE IS NO Price chgble. at service entry

4) item category - PT9 - Price and value fields - Price chgble. at service entry - Optional

5) functional authorization - EFB - X maintained

6) sub item -  not required for service as u said

Regards