Why can't void be used in this context?

No you can not use void in void(method in method). You check if player entered grass land, but now you want to tell other methods that player entered grass land. Just create new variable

 bool enter = false;

So when player enter grass land set it to true, when he leave set it to false.

 using System.Collections;
  
     public class DetectGridGrassland : MonoBehaviour {
  
     public int randNum = 0;
     public bool enter = false;
     void OnTriggerEnter(Collider other){
        Debug.Log("Player entered the Grass Land");
        enter = true; //We entered grass land, set enter to true
      }
     void OnTriggerExit(Collider other){
         enter = false;//We left grass land, set enter to false
     }
        void Start(){
          randNum = Random.Range(0,5);
        }
  
        void OnGUI(){
         //Check if enter is true
         if(enter == true){
          if (randNum <= 3)
          {
           GUI.Button(new Rect(10, 10, 125, 25), "Attacked by bandits");
          }
          else if(randNum == 3)
          {
           GUI.Button(new Rect(10, 10+20, 125, 25), "Attacked by wolves");
          }
          else
          {
           GUI.Button(new Rect(10, 10+30, 125, 25), "Nothing to do!");
          }
        }
     }
 }