Hi,
If you're trying to assign the profit center field and the FM is active with PSGLFLEXT, you won't be able to do it because, SAP understands the Public Sector don't used profit center.
If your client isn't Public Sector, you can active FM, but before this, what you should have to do is enabled Document Splitting for all companies code.
Check the note: 891144 - New GL - Document splitting - Risks with subsequent changes. Maybe this can help you.
JPA