enable/disable script from another script c#?

Instead of

     public Component N20;
     public Component Setup;

You could give these fields the type they are (which is probably what you wanted in the first place).

Here's how:

     public N20 n20;
     public Setup setup;

N20 is the class name (type), n20 is the name we are giving this variable.

And then you would use

     //.enabled instead of .enable
     n20.enabled = true;

to enable the script.

Note 1: You will also have to change your Start function (as the names of the fields have changed (N20 is now n20)).

Note2: By convention class names start with a capital letter, for better readability consider changing "public class nitro" into "public class Nitro".

You must declare a script as MonoBehaviour (base class) or as its actual type, which's just the script's file name without any extension - and never give to a variable a name equal to some existing type (this may cause incredible confusions!). Furthermore, the property that enables a script is enabled, not enable.

Supposing that the two scripts are called N20.cs and Setup.cs, for instance, you should write something like this:

 using UnityEngine;
 using System.Collections;
  
 public class nitro : MonoBehaviour {
  
   public N20 scriptN20;
   public Setup scriptSetup;
  
   void Start(){
     scriptN20 = GetComponent<N20>();
     scriptSetup = GetComponent<Setup>();
   }  
  
   void Update(){
     if(Input.GetButtonDown("nitro")){ 
       scriptN20.enabled = true;
       scriptSetup.enabled = false;  
     }
     else
     if(Input.GetButtonUp("nitro")){ 
       scriptN20.enabled = false;
       scriptSetup.enabled = true;   
     }
   }

 }