The input network dataset does not define a cost attribute named WalkTime.
The WalkTime cost attribute must be available on the input network dataset for the tools in the NetworkAnalysis and NetworkAnalysisSync geoprocessing services to run as expected. If this cost attribute is missing, the tools use the first cost attribute defined on the network dataset, which may cause unexpected analysis results.
If your network dataset can model the time taken by pedestrians to walk on the streets, define a cost attribute named WalkTime on the network dataset. Otherwise, do not use the Walk Time option when specifying the Impedance parameter on the tools.