Error add-Migration . net core 3

Question

I am trying to add the add-Migration in my solution but always happens the following error "Value cannot be null (Parameter 'connectionString')"

but my connection string is correct in appsettings.json

  public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc();
    services.AddDbContext<SimCoContext>(optBuilder =>
        optBuilder.UseNpgsql(Configuration.GetConnectionString("DefaultConnection")));
}

Appsettings.json

{
  "ConnectionsStrings": {
    "DefaultConnection": "Host=localhost;Port=5432;Pooling=true;User ID=postgres;Password=admin;Database=SimCo;"
  },
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft": "Warning",
      "Microsoft.Hosting.Lifetime": "Information"
    }
  },
  "AllowedHosts": "*"
}

Answer 1

Already checked your appsetting.Development.json?

By default, when ASPNETCORE_ENVIRONMENT is not set in its environment variables, it uses the Development configuration.

More in: https://docs.microsoft.com/en-us/aspnet/core/fundamentals/environments?view=aspnetcore-3.1

Answer 2

public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc();
    services.AddDbContext<SimCoContext>(optBuilder =>
        optBuilder.UseNpgsql(Configuration.GetConnectionString("DefaultConnection")));
}

Appsettings.json

{
  "ConnectionsStrings": {
    "DefaultConnection": "Host=localhost;Port=5432;Pooling=true;User ID=postgres;Password=admin;Database=SimCo;"
  },
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft": "Warning",
      "Microsoft.Hosting.Lifetime": "Information"
    }
  },
  "AllowedHosts": "*"
}