bool isSceneExists(string sceneName)
{
    for (int i = 0; i < SceneManager.sceneCountInBuildSettings; i++)
    {
        string path = SceneUtility.GetScenePathByBuildIndex(i);
        string name = System.IO.Path.GetFileNameWithoutExtension(path);

        if (name == sceneName)
            return true;
    }
    return false;
}


// And then Just
if (isSceneExists("yourSceneName"))
{
    SceneManager.LoadScene("yourSceneName");
}
else
{
    // what you want
}