Available with Network Analyst license.
The network dataset does not have a default cost attribute.
The cost of traveling along a network element is modeled using cost attributes on the network dataset. The most common cost attributes model driving time or distance. When finding the best route or solving other types of network analyses, the values of a cost attribute are required because the solver minimizes them in some way. The network dataset needs at least one default cost attribute.
Set a default cost attribute and rebuild the network dataset.
To set a default cost attribute, open the Network Dataset Properties window from the Catalog window, click the Attributes tab, right-click a cost attribute, and choose Use By Default.