If you want to destroy the game object, please call 'Destroy' on the game object instead.

I take it you're pretty new to Unity and coding in general, so I'll try to break this down as much as I can. First I think you need some clarification on the relationship between a GameObject and a Transform.

The pedestrian prefab you're using is a GameObject, and all GameObjects have a Transform component attached to them (just like any other component, like a RigidBody, Collider, etc.).

When you Instantiate the Pedestrian in your code, you're giving it three arguments: the prefab to instantiate ["Ped"], what position you want it to be instantiated at (meaning, the position variables of its Transform) [Vector3 (0, 0, -2)], and what rotation you want it to be instantiated with [Quaternion.Identity]

So in order to move your pedestrian GameObject, you'll need to change the position variables of its Transform component. Nothing in your code above will move the pedestrian, since you never change its position, nor will it ever be destroyed (since it's distance from this SpawnPerson object will never reach > 90). In fact, that part of your code will never evaluate correctly, since distance is always positive (you can't be negative X metres away from something, right?), so you'll have to change it to 90, rather than -90. On top of that, each time you Instantiate a new Pedestrian, clonePed will change to only have a reference to the newest one you've created, so when you check the distance later, you're actually only checking the distance between this object and the latest pedestrian you've created (all the other ones will continue to exist forever!).

So, first thing first:

Instantiating Properly

 var clonePed = Instantiate (Ped, new Vector3 (0, 0, -2), Quaternion.identity);

So first a bit of a gotcha -- using var isn't that helpful when you're first starting. Normally, when you make a variable, you give it an explicit type, like float, or string. With var, you're essentially saying "make this variable whatever type it needs to be when something is assigned to it". If you created the following variable:

 var number = 10;

Do you know now whether number would be an Int or a Float? You might guess, but why take the risk and guess wrong? So, let's instead ensure that "clonePed" is explicitly a GameObject. Change the line to:

 GameObject clonePed = Instantiate (Ped, new Vector3 (0, 0, -2), Quaternion.identity) as GameObject;

The "as GameObject" declaration at the end is important, because otherwise it'll simply get Instantiated as an Object, rather than a GameObject, as that's what the Instantiate function does (don't worry about this distinction too much right now, just know that it's important!).

Next, at the very top, we need to change:

 public Transform Ped;

to

 public GameObject Ped;

Instantiate wants you to pass it a GameObject in order for it to create something, so you can't pass it a Transform. Once you've changed this line, if you go into the editor and select the object you've got your SpawnPerson script attached to, you'll see a "Ped" field has been added to it. You're going to want to drag your pedestrian prefab from your assets list into this slot in order to assign the prefab to that variable.

Moving the Pedestrian

In order to do this, I'd suggest adding a new script to your pedestrian prefab, call it PedestrianMovement:

 public class Pedestrian : MonoBehaviour
     {
         Vector3 startingPosition;
         float speed = 5;
         float distanceToTravel = 5;
 
         void Start ()
         {
             // Get the starting position of the pedestrian so we can compare the distance
             startingPosition = transform.position;
         }
 
         void Update ()
         {
             // If the distance the pedestrian has traveled from its starting position is less than 5, move it
             if (Vector3.Distance (startingPosition, transform.position) < distanceToTravel)
             {
                 transform.position = new Vector3 (transform.position.x + speed * Time.DeltaTime, transform.position.y, transform.position.z);
             }
             
             // Otherwise, if the distance the pedestrian has traveled is greater than 5, destroy it
             else
             {
                 Destroy (gameObject);
             }
         }
         
     }

That script will run when the Pedestrian prefab is Instantiated, and it'll begin moving immediately (I just have it set to move to the right). Once it's moved far enough it'll get destroyed. When calling Destroy, using the variable gameObject (lowercase G) as I have, essentially means "the GameObject that this script is attached to". I've changed the distance to travel to 5 so you can see the effects more immediately without having to wait as long (feel free to change the value of the distanceToTravel variable).

Finally, I'm not sure what your "x" variable is tracking, but as it is now it's going to increase both when a pedestrian is spawned, and every frame as well (the "else" statement). If you can clarify what you're trying to do with this I can try to help with that as well.

I highly recommend starting with some very basic Unity tutorials (either written or on YouTube), since I think it'll make the process of learning a lot less frustrating (since you spent all day hunting down what errors mean without any context!).

Hope that helps!