Available with Network Analyst license.
Network dataset does not have a time attribute. A cost attribute with a time unit is required on a network dataset to generate directions and may be needed for other functionality as well.
Take any one of the following actions to resolve this issue:
Do nothing. Mark this warning as an exception to avoid seeing it again.
Open the Network Dataset Properties window, click the Attributes tab, and create a cost attribute to model time. (Prior to setting up the cost attribute, you may need to add a time field to your source feature classes.)