Description
When specifying the values for the field_mappings parameter, the field mappings for the Locations property cannot be specified along with field mappings for the SourceID, SourceOID, PosAlong, and SideOfEdge properties.
Solution
If you are adding features using network location ranges, specify the field mappings only for the Locations property. Do not specify the field mappings for the network location fields (SourceID, SourceOID, PosAlong, SideOfEdge).