I have got a problem with this kod. Can you help me?

I guess your problem is, that NavMeshAgent is not found…?
I also guess you want to use Unity - Scripting API: NavMeshAgent , which appearantly is in the namespace UnityEngine.AI, so you have to add the line

using UnityEngine.AI;

At the top.

Note: Your IDE usually can help you with those things.

Your GetComponent is wrong. It should be:

GetComponent<NavMeshAgent>();