https://userapps.support.sap.com/sap/support/knowledge/en/2918802
2918802 - Extension fields not available on API API_FINWBSELEMENT_SRV and API_FINPROJECT_SRV
You want to use extension fields, for example WBS Element, for API_FINWBSELEMENT_SRV and API_FINPROJECT_SRV, but this is not possible.
extension fieldsnot available