c# Api RestClient (namespace name "RestClient" could not be found )

Question

I'm trying to establish a connection to my banker's server using RestClient API. The code they provided is below, but the Restclient is not working. I'm not going to post the API_key here but How do I fix this issue. In the C# program, RestClient is underlined with this error message: "C20246: The type or namespace name 'RestClient could not be found." I installed RestSharp and put using RestSharp; in the top then a lot of errors CS1061 saying RestClient does not contain definition.

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)
        {
            restClient.DeleteOrderAsync(order.OrderId);
        }

        // Figure out when the market will close so we can prepare to sell beforehand.
        var calendars = restClient.ListCalendarAsync(DateTime.Today).Result;
        var calendarDate = calendars.First().TradingDate;
        var closingTime = calendars.First().TradingCloseTime;
        closingTime = new DateTime(calendarDate.Year, calendarDate.Month, calendarDate.Day, closingTime.Hour, closingTime.Minute, closingTime.Second);

        Console.WriteLine("Waiting for market open...");
        AwaitMarketOpen();
        Console.WriteLine("Market opened.");
    }