Matchmaking error when user Server RPC call to Client

Unity Engine
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1

It got this problem when connecting and use Server RPC call to Client to matchmaking
Assets\Scripts\FindMatchScene\MatchmakingButton.cs(35,9): error CS0103: The name ‘MatchmakingManager’ does not exist in the current context

using Unity.Netcode;
using UnityEngine;
using UnityEngine.UI;

public class MatchmakingButton : NetworkBehaviour
{
    public Button findMatchButton;

    private void Start()
    {
        if (findMatchButton != null)
        {
            findMatchButton.onClick.AddListener(OnFindMatchButtonPressed);
        }
        else
        {
            Debug.LogError("Button not assigned!");
        }
    }
    public void OnFindMatchButtonPressed()
    {
        if (NetworkManager.Singleton.IsClient)
        {
            // Gửi tín hiệu tới server để bắt đầu tìm trận
            RequestMatchmakingStartServerRpc();
        }
    }

    [ServerRpc(RequireOwnership = false)] 
    private void RequestMatchmakingStartServerRpc(ServerRpcParams rpcParams = default)
    {
  
        MatchmakingManager.Instance.StartMatchmakingLogic();
    }
}