Inheritance from object to object

I have been working with inheritance in classes for the first time now and some things confuse me. Say I define some variables at the top of the class, give them some value in the Start() function and use them in a sub-class. Sometimes my variables cannot be found in the subclass.

For example, in the base class I type this:

 using UnityEngine;
 using System.Collections;
 
 public class GeneralItemScript : MonoBehaviour {
     
     public InteractionScript iaScript;
     public GameObject player;
     
     // Use this for initialization
     void Start () {
         player = GameObject.Find ("Player");
         
         iaScript = player.GetComponent<InteractionScript>();
         
     }
     
     // Update is called once per frame
     void Update () {
     
     }
     
     
     virtual public void PerformInteraction() {
         
     }
 }

And in the subclass I attempt to use this iaScript in a custom function ( PerformInteraction() ):

 using UnityEngine;
 using System.Collections;
 
 public class PickupItemScript : GeneralItemScript {
     
     public bool b_isCarried;
     
     // Use this for initialization
     void Start () {
     
         b_isCarried = false;
         
     }
     
     // Update is called once per frame
     void Update () {
         
     }
     
     override public void PerformInteraction() {
         if (iaScript.SetCarriedItem(this.gameObject)) {
             b_isCarried = true;
         }
     }
     
     
     public void SetCarryLocation() {
         transform.parent = player.transform.Find("Arm");
     }
     
     public void StopCarry() {
         Vector3 tempVec = transform.position;
         transform.parent = null;
         transform.position = tempVec;
         b_isCarried = false;
     }
 }

This earns me the error that I cannot find the object referenced. What are the general rules/ do's and don'ts of inheritance? And how does overriding functions work exactly?

EDIT: added entire scripts