The network dataset does not have a length attribute. A cost attribute with a length unit is required on a network dataset to generate directions and may be needed for other functionality as well.
Solutions
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 length. (Prior to setting up the cost attribute, you may need to add a length field to your source feature classes.)