I would suggest not trying to use inline code like that. That style is old classic ASP style and I'm not even sure it fully works in ASP.Net. First put runat="server" on your a tag and id="hypSip" so you can access it in C# or use an asp:HyperLink instead.
In your Page_Load event in C# codebehind page you can simply do
hypSip.Visible = Session["Rol"].ToString() == "Yönetici";