Hi Amit,
When you create the production order manually through CO01, planned order field will be greyed out. this is standard SAP behavior.
You have following options.
1. Apply relevant use exit's while saving the production order, which checks for the value in planned order and if field is empty it will not allow saving of the production order. Take the help from ABAP consultant.
2. Remove the authorization of CO01 for users and give authorization for T-codes MD04 or CO40 or CO41 so that they can create production orders by converting planned orders only.
Hope this helps..
Thanks
Kumar