Network dataset uses live traffic. Live traffic isn't supported in runtime; however, historical traffic is.
Either remove live traffic from the cost attribute's edge traffic evaluator or remove the cost attribute entirely.
Open the Network Dataset Properties dialog box, click the Traffic tab, and change the Table property in the Live Traffic: Streets-TMC Table section to <None>.
Alternatively, open the Network Dataset Properties dialog box, click the Attributes tab, choose the attribute that includes live traffic, and remove the attribute.