List does not contain a definition for Except error.

Hello! I'm getting the following error:

'List' does not contain a definition for 'Except' and no extension method 'Except' accepting a first argument of type 'List' could be found (are you missing a using directive or an assembly reference?)

These are the lines of code I'm trying to use:

         List<GameObject> allPoints = new List<GameObject>(GameObject.FindGameObjectsWithTag("MountPoint"));
         otherMountingPoints = allPoints.Except(thisMountingPoints).ToList();

I got the following imports:

 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections.Generic;

Does anyone have an idea on why the method Except can't be found? I've looked around the web and couldn't find anything. Thank you.