Consolidated Bill Group cannot assign at Task

I have setup the Consolidated Bill Group and selected usage 'Projects'

I have setup Project type contract and assign the same customer at Project level for which Consolidated Bill Group is being setup

Requirement is to assign Consolidated Bill Group at top task level , but at Billing setup the Consolidated Bill Group is field is disabled ?? so cannot assign it

also i have checked Billing setup at Project level also and the Consolidated Bill Group is field is disabled ?? so cannot assign it

please assist is there are any setup missing??

Thanks