can't start Coroutine

[Contact1]

Hello I can’t start the line where it says StartCoroutine(RespawnCo()); it’s highlighting red can anyone help me please Thank you!!!

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

public class LevelManager : MonoBehaviour
{
    public static LevelManager instance;

    public float waitToRespawn;

    private void Awake()
    {
        instance = this;
    }
    // Start is called before the first frame update
    void Start()
    {
       
    }

    // Update is called once per frame
    void Update()
    {
       
    }

    public void RespawnPlayer()
    {
        StartCoroutine(RespawnCo());
    }

    private IEnumerable RespawnCo()
    {
        PlayerController.instance.gameObject.SetActive(false);

        yield return new WaitForSeconds(waitToRespawn);

        PlayerController.instance.gameObject.SetActive(true);
    }
}

[system]

@Contact1

private IEnumerable RespawnCo()

IEnumerable → IEnumerator

Please read this: https://docs.unity3d.com/Manual/Coroutines.html