hi Thang,
What do you mean by "commitment item from valuation class and fund center"?? Means CI from combination of valuation class + fund center.
If you are saying that you have activated step 3 with overwrite feature and still you are not able to derive commitment item, it means you are saying that no GL is deriving in case of PR with account assignment "K".
It's confusing. Review your derivation rules again you will find the solution.
Regards,
Ankit K. Agarwal