@Dalux_Chen, see if the below link helps. CostCenter is not a value stored on the sales order. It is part of the financial dimension. It is converted to specific dimension format & stored in a different table. You need to extract value of specific dimension from that. Below link has sample code for it.
https://community.dynamics.com/blogs/post/?postid=e3181cc3-0c22-4005-b0d6-cd0080a4a569