Need help with knockback in C# Please!

You can use AddForce, but only if the enemy is a rigidbody (if it's a CharacterController, a different approach must be used)
For a melee, we must somehow calculate the direction of the impact: a simple solution is to use the direction melee->enemy, but ignoring the vertical component (pushing the enemy against the ground usually doesn't produce a decent effect).
That's an example (attach this to the melee):

   public float force = 500; // adjust the impact force
 
   void OnTriggerEnter(Collider other){
     Vector3 dir = other.transform.position - transform.position;
     dir.y = 0; // keep the force horizontal
     // try to get rigidbody of other object
     Rigidbody rb = other.GetComponent<Rigidbody>();
     if (rb){ // use AddForce for rigidbodies:
       rb.AddForce(dir.normalized * force);
     } else { // use a special script for character controllers:
       // try to get the enemy's script ImpactReceiver:
       ImpactReceiver script = other.GetComponent<ImpactReceiver>();
       // if it has such script, add the impact force:
       if (script) script.AddImpact(dir.normalized * force);
     }
   }

If the enemy is a CharacterController, you must add the script ImpactReceiver below to its prefab, so that all enemies will be able to receive impacts like rigidbodies do:

 using UnityEngine;
 
 public class ImpactReceiver: MonoBehaviour {
   
   public float mass = 3.0; // define the character mass
 
   Vector3 impact = Vector3.zero;
   CharacterController character;
 
   void Start(){
     character = GetComponent< CharacterController>();
   }
 
   void AddImpact(Vector3 force){ // CharacterController version of AddForce
     impact += force / mass;
   }
 
   void Update(){ 
     // apply the impact effect:
     if (impact.magnitude > 0.2f){
       character.Move(impact * Time.deltaTime);
     }
     // impact energy goes by over time:
     impact = Vector3.Lerp(impact, Vector3.zero, 5*Time.deltaTime);
   }
 }

NOTE: If the melee is the trigger, add a kinematic rigidbody to it - moving triggers need it to work fine.