заменил var doc = new HtmlDocument(); на var doc = new HtmlAgilityPack.HtmlDocument();
Теперь появилась другая ошибка
"The type 'System.Xml.XPath.IXPathNavigable' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Xml, Version=4.0.0.0, Culture=neutral...