[System.Serializable]
public class TextOfTweet
{
    public string text;
}

[System.Serializable]
public class RootObject
{
    public List<TextOfTweet> tweetTexts;
}

public class MoveTweetDown : MonoBehaviour {
    private int thisSpeed;

    private void OnEnable()
    {
        thisSpeed = LevelManager.current.speedOfTweets;
        string path = Application.streamingAssetsPath + "/Covfefefiltered.json";
        string jsonString = File.ReadAllText(path);
        RootObject textForTweet = JsonUtility.FromJson<RootObject>(jsonString);
        Debug.Log(textForTweet.tweetTexts[0].text); //Error ArgumentOutOfRangeException here
        int randomTweetN = Random.Range(0, textForTweet.tweetTexts.Count);

        gameObject.GetComponentInChildren<TextMesh>().text = textForTweet.tweetTexts[randomTweetN].text;
    }