Solution : https://service.sap.com/sap/support/notes/135449 (SAP Service marketplace login required)
Key words :
system issues error message vf155, additional key words wbs element, leading wbs element & determined, project structure element, attached advance correction, program error, account assignment, substitution, prerequisites
Related Notes :
| 157321 | Order-rel. billing:Check for WBS element is missing |