"Price change allowed" Check box in Service PO grayed out

Hi Experts,

I am new to MM. I am trying to do the settings for service PO so that the gross price can be changed at SES level. In order to meet this requirement I have made settings in SPRO -> MM -> External Service Management -> Define screen layout -> 4. Purchase order--> price and value fields--> price change allowed as optional..

After this setting I am able to see the "Price Change allowed" column in PO under the service tab. However it is grayed out and I am unable to set the check box.

Am i missing on any settings? Please help.

Thanks

Regards,

DDP