Error executing add-Migration . Entity Framework Core

Question

I am using version 2.2.4 of Ef core and version 3.0.5 preview of . net core sdk

The mistake is : Unable to create an object of type 'AppDbContext'. For the different patterns supported at design time, see https://go.microsoft.com/fwlink/?linkid=851728 When I try to generate a database migration script.

startup.Cs

using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using APICatalogo.Context;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.HttpsPolicy;
using Microsoft.AspNetCore.Mvc;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;

namespace APICatalogo
{
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {

            services.AddDbContext<AppDbContext>
                (options => options.UseMySql(Configuration.GetConnectionString("DefaultConnection")));
            services.AddControllers()

My db context class:

using APICatalogo.Models;
using Microsoft.EntityFrameworkCore;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace APICatalogo.Context
{
    public class AppDbContext : DbContext
    {

        public AppDbContext(DbContextOptions<AppDbContext> options)
        : base(options)
        {
        }
        public DbSet<Categoria> Categorias { get; set; }
        public DbSet<Produto> Produtos { get; set; }

    }
}