C# 246 error with RESTCLIENT in example code

Visual Studio
c# .NET 6.0
Alpaca SDK 6.05

Wanted to try out this API and check out this example: A Simple Mean Reversion Stock Trading Script in C#

wanted to see if I could adapt the code to what I need but wont run due to error and cant find a way around it

class MeanReversionPaperOnly
private string API_KEY = “REPLACEME”;
private string API_SECRET = “REPLACEME”;
private string API_URL = “https://paper-api.alpaca.markets”;

    private string symbol = "SPY";
    private Decimal scale = 200;

   private ***RestClient*** restClient;

    private Guid lastTradeId = Guid.NewGuid();
    private List<Decimal> closingPrices = new List<Decimal>();

    public void Run()
       restClient = new ***RestClient***(API_KEY, API_SECRET, API_URL);

        // First, cancel any existing orders so they don't impact our buying power.
        var orders = restClient.ListOrdersAsync().Result;
        foreach (var order in orders)