i cant fix a error

how to fix this thing?

Assets/Standard Assets/Characters/ThirdPersonCharacter/Scripts/AICharacterControl.cs(10,16): error CS0246: The type or namespace name NavMeshAgent' could not be found. Are you missing UnityEngine.AI’ using directive?

You probably tried to declare a NavMeshAgent or the script (wich is from the standard assets) did.
Recently every class concerning Navigation got moved to the ‘UnityEngine.AI’ namespace. To declare a NavMeshAgent variable you either have to declare it with the whole namespace like this :

private UnityEngine.AI.NavMeshAgent agent;

Or you add a using directive at the top of the concerned script and the namespace will be used implicitly, and fix every call to any NavMesh classes :

using UnityEngine.AI;