Hello Kareemulla,
Can you please check master data for your activity
t'code KL03
Check what is the activity type category (Is it Activity Type Category 2)
If yes, please confirm are you using Plan Indirect activity allocation (KSCB), because Activity type category 2 works on Indirect calculation, indirect allocation. Category 2 activities are used in indirect activity allocation and the price of these activities are calculated automatically through indirect activity allocation with the help of relationship between sender & receiver cost centers, or receiver tracing factor, or fixed quantities defined in segments. Here plan activities are automatically reconciled with scheduled activities.
If you are not using Indirect activity allocation then change the type to either 1 or 3 (depending upon your situation)
Please check the below doc it will be helpfull.
http://scn.sap.com/docs/DOC-57434
Best regards,
Manish