could not find a part of the path 'c:\program files (x86)\iis express\ code example

Example: could not find a part of the path 'c:\program files (x86)\iis express\

var path = Server.MapPath(@"~/TextFiles/ActiveUsers.txt");

using (System.IO.StreamWriter file = new System.IO.StreamWriter(path, true))
{
    file.WriteLine(model.UserName.ToString());
}