"ELEMENT 'Textbox' is not a known element This may occur if there is a compilation error or web.config is missing".i am not able to change properties

Hi I have a textbox and other controls in my form. The problem is I get a greenline underlining my controls saying "ELEMENT 'Textbox' is not a known element This may occur if there is a compilation error or web.config is missing" But the project is running fine. Where should I look to solve this error?

                         <div class="col-md-6">
                            <label>Book Id</label>
                            <div class="form-group">
                                <div class="input-group">
                                <asp:TextBox CssClass="form-control" ID="TextBox1" runat="server"
                                    placeholder="Id"></asp:TextBox>
                                  <asp:Button CssClass="btn btn-primary " ID="Button1" runat="server"
                                        Text="Go" />
                                     </div>
                            </div>
                        </div>


                    </div>