Serializing pickups?

Hi, lets say I have quite a good amount of collectables

• I want to place the prefabs as I wish on the scene and have each of them have their own index

• I want them to be saved, so when I enter on the same level, they appear on different color to indicate they've been collected

• I want them to interact with the UI, both in the level and out of it, to show which ones are already collected

I got this code in a script

 using System.Linq;
 using UnityEngine;
 
 /// <summary>
 /// Behaviour for all collectables.
 /// </summary>
 [ExecuteAlways]
 public class CollectableBehaviour : MonoBehaviour {
     /// <summary>
     /// The tag used to search for collectables
     /// </summary>
     private const string CollectableTag = "Collectable";
     private static int currentSerializatonId = -1; 
     
     /// <summary>
     /// Static initializer.
     /// </summary>
     static CollectableBehaviour() {
         if (currentSerializatonId == -1) {
             var lastCollectable = GameObject.FindGameObjectsWithTag(CollectableTag)
                 .Select(o => o.GetComponent<CollectableBehaviour>())
                 .Where(o => o != null)
                 .OrderByDescending(o => o.serializatonId)
                 .FirstOrDefault();
             currentSerializatonId = lastCollectable == null ? 0 : lastCollectable.serializatonId + 1;
         }
     }
 
     /// <summary>
     /// The unique identifier for this collectable.
     /// </summary>
     public int serializatonId = -1;
 
     /// <summary>
     /// Called to initialize this behaviour.
     /// </summary>
     void Awake() {
         if (serializatonId == -1) {
             serializatonId = currentSerializatonId++;
             gameObject.tag = CollectableTag;
         }
     }
 }

Then, the pickup object has this code:

public class CoinController : CollectableBehaviour { //Do something }

 UnityException: FindGameObjectsWithTag is not allowed to be called from a MonoBehaviour constructor (or instance field initializer), call it in Awake or Start instead. Called from MonoBehaviour 'CoinController' on game object 'Cylinder'.
 See "Script Serialization" page in the Unity Manual for further details.
 CollectableBehaviour..cctor () (at Assets/Script/CollectableBehaviour.cs:23)
 Rethrow as TypeInitializationException: The type initializer for 'CollectableBehaviour' threw an exception.