Call function of a MonoBehaviour class in another class

So i am trying to call a function in this class: using UnityEngine; using UnityEngine.UI; using System.Collections;

 public class AchievementDisplay : MonoBehaviour {
 
     Text text;
 
     void Start()
     {
         gameObject.SetActive(true);
         text = GetComponent<Text>();
        
     }
     
     
     void Update () {
     
     }
 
    public void achievementActivate(string achText) {
         Debug.Log("PLEB");
         text.text = achText;
         moveUp();
 
     }
 
     void moveUp() {
         var t = 0;
         while (t < 100) {
             transform.Translate(Vector3.up * Time.deltaTime);
         }
         Invoke("moveDown", 5);
     }
 
     void moveDown() {
         var t = 0;
         while (t < 100)
         {
             transform.Translate(Vector3.down * Time.deltaTime);
         }
     }
 }
 

via this class: using UnityEngine; using System.Collections; using System.Collections.Generic; using System;

 public class AchievementManager : MonoBehaviour {
 
     // This class is used to check for achievements and give. I have no idea how to do this. Help would be cool.
 
     List<Achievement> ach = new List<Achievement>();
     List<Achievement> activatedAch = new List<Achievement>();
 
     AchievementDisplay achievedis = new AchievementDisplay();
 
 
     void Start () {
         ach.Add(new Achievement(2, "clicks", "poopie"));
     }
 
     void Update () {
         foreach(Achievement achie in ach){
             if (achie.theSomething == "clicks"){
                 if(achie.amountOfSomething == Data.clicks){
                     achie.activated = true;
                 }
             }
 
             if (achie.activated) {
                 achie.activated = false;
                 activatedAch.Add(achie);
                 achievedis.achievementActivate(achie.text);
                 ach.Remove(achie);
             }
         }
     }
 }

But i am getting this error: You are trying to create a MonoBehaviour using the 'new' keyword. This is not allowed. MonoBehaviours can only be added using AddComponent(). Alternatively, your script can inherit from ScriptableObject or no base class at all UnityEngine.MonoBehaviour:.ctor() AchievementDisplay:.ctor() AchievementManager:.ctor()

Is there a good way to do this?