“SetDestination” can only be called on an active agent that has been placed on a NavMesh.

hello, I have problems with a script the error is above in the title and these are my scripts

Code (CSharp):

1. public class spawnunits : MonoBehaviour
2. {
3.     public int crtUnits;
4.     public GameObject units;
5.     public woodunit wu;
6.     public Vector3 vec;
7.  
8.     public void Start()
9.     {
10.         crtUnits = 0;
11.         StartCoroutine(CoUpdate());
12.     }
13.  
14.     IEnumerator CoUpdate()
15.     {
16.        
17.         if (crtUnits < 20)
18.         {
19.             yield return new WaitForSeconds(1);
20.             Instantiate(units, transform.position, Quaternion.identity);
21.             yield return new WaitForSeconds(1);
22.             Instantiate(units, transform.position, Quaternion.identity);
23.             yield return new WaitForSeconds(1);
24.             Instantiate(units, transform.position, Quaternion.identity);
25.  
26.         }
27.         wu.GetWood();
28.        
29.         yield return null;
30.     }
31.    
32. }

Code (CSharp):

1. public class woodunit : MonoBehaviour
2. {
3.     public GameObject treedest;
4.     public GameObject homebase;
5.     public NavMeshAgent unitagent;
6.     public void Update()
7.     {
8.        
9.     }
10.     public void GetWood()
11.     {
12.         treedest = GameObject.Find("treeparent(Clone)");
13.  
14.         unitagent.SetDestination(treedest.transform.position);
15.     }
16.     public void ReturnHome()
17.     {
18.        
19.     }
20.    
21. }
22.  

What they should do is that the first script should instantiate 3 Navmesh Units ( it does that good )
then the second should set the units' location thru code towards a tree, my trees are randomly instantiated in the scene so I can't assign them in the inspector even If I wanted to, anyway after the 3 Navmesh Units spawn I get this

“SetDestination” can only be called on an active agent that has been placed on a NavMesh.

I've assigned everything correctly ( I hope ), I've used UnityEngine.AI, at first the script worked only when I told the units the location in a void Start() but when I do it in a custom function it does not work, also it does not work in void Update. any clue what can cause this? I work in 2D with a 2D NavMesh Plugin ( it is the same as a normal navmesh but rotated on the XY( like a wall) axis instead of XZ ( like a floor).