Default is not Allowed here because it does not extend class System | Server Parser Error in ASP.NET

solution with detailed explanation

what can i do, if i see that all the code is good but the error persist?

i just created a new web form, and the syntaxis is correct but this problem persist, and i don't know why: Line 1: <%@ Page Language="C#" AutoEventWireup="true" CodeBehind="WebForm7.aspx.cs" Inherits="GestorDatosServimed.WebForm7" %>