Any ideas as to why I get this error? (CS1061)

This is because you are accessing the renderer via the deprecated property "renderer" this property is now of type "Component" not of type "Renderer" This was done alongside a load of other similar properties like rigidbody to reduce coupling in the unity code base and make things a bit more modular.

What you need to do is either cast renderer to an appropriate type, or better yet, store an appropriate reference to it as a variable using GetComponent. for example. (Code done in notepad, may be slight syntax errors).

 using UnityEngine;
  
  [AddComponentMenu("Rendering/SetRenderQueue")]
  public class SetRenderQueue : MonoBehaviour {
  
      [SerializeField]
      protected int[] m_queues = new int[]{3000};
      
      private MeshRenderer m_renderer;
      
      protected void Awake(){
        m_renderer = GetComponent<MeshRenderer>();
          Material[] materials = m_renderer.materials;
          for (int i = 0; i < materials.Length && i < m_queues.Length; ++i){
              materials[i].renderQueue = m_queues[i];
          }
      }
  }