Hi Sharvari Joshi,
Your question is little bit confusing, if you have not found out how to configure "true postings for cost center as required", open your order type and at the bottom of the screen you find a button named with FIELD SELECTION like below...
If You have already marked cost center: true postings as required entry like below....
Then, please go to the creation mode of order (KO01), there under control data you find the results of above setting, which you can see below..
Make sure, you internal order is statistical either here or defaulted from order type.
Regards...
Jose