Entity Framework + PostgreSQL code-first

First time here. I'm trying to integrate Entity Framework with PostgreSQL using Visual Studio 2017 Community:

From Nuget, I installed this packages:

• EntityFramework.6.1.3
• Npgsql.3.2.2
• EntityFramework6.Npgsql.3.1.1

My project is a Web API with .NET Framework 4.6.2 (not core)

My web.config looks like this

<system.data>
    <DbProviderFactories>
        <remove invariant="Npgsql"/>
        <add name="Npgsql Data Provider" invariant="Npgsql" support="FF" description="Data Provider for PostgreSQL" type="Npgsql.NpgsqlFactory, Npgsql, Version=4.0.2.0, Culture=neutral, PublicKeyToken=5D8B90D52F46FDA7"/>
    </DbProviderFactories>
</system.data>
<entityFramework>
    <defaultConnectionFactory type="Npgsql.NpgsqlConnectionFactory, Npgsql"/>
    <providers>
        <provider invariantName="Npgsql" type="Npgsql.NpgsqlServices, EntityFramework6.Npgsql"/>
    </providers>
</entityFramework>

But, in the Nuget Packet Manager's console, after issuing Enable Migrations, I get this error:

Checking if the context targets an existing database...
System.Configuration.ConfigurationErrorsException: Failed to find or load the registered .Net Framework Data Provider.

in System.Data.Common.DbProviderFactories.GetFactory(DataRow providerRow)
in System.Data.Common.DbProviderFactories.GetFactory(String providerInvariantName)
in System.Data.Entity.Infrastructure.DependencyResolution.DefaultProviderFactoryResolver.GetService(Type type, Object key, Func3 handleFailedLookup)
in System.Data.Entity.Infrastructure.DependencyResolution.DefaultProviderFactoryResolver.GetService(Type type, Object key)
in System.Data.Entity.Infrastructure.DependencyResolution.CachingDependencyResolver.<>c__DisplayClass1.<GetService>b__0(Tuple2 k)
in System.Collections.Concurrent.ConcurrentDictionary2.GetOrAdd(TKey key, Func2 valueFactory)
in System.Data.Entity.Infrastructure.DependencyResolution.CachingDependencyResolver.GetService(Type type, Object key)
in System.Data.Entity.Infrastructure.DependencyResolution.ResolverChain.<>c__DisplayClass3.b__0(IDbDependencyResolver r)
in System.Linq.Enumerable.WhereSelectArrayIterator2.MoveNext()
in System.Linq.Enumerable.FirstOrDefault[TSource](IEnumerable1 source, Func2 predicate)
in System.Data.Entity.Infrastructure.DependencyResolution.ResolverChain.GetService(Type type, Object key)
in System.Data.Entity.Infrastructure.DependencyResolution.RootDependencyResolver.GetService(Type type, Object key)
in System.Data.Entity.Infrastructure.DependencyResolution.ResolverChain.<>c__DisplayClass3.<GetService>b__0(IDbDependencyResolver r)
in System.Linq.Enumerable.WhereSelectArrayIterator2.MoveNext()
in System.Linq.Enumerable.FirstOrDefault[TSource](IEnumerable1 source, Func2 predicate)
in System.Data.Entity.Infrastructure.DependencyResolution.ResolverChain.GetService(Type type, Object key)
in System.Data.Entity.Infrastructure.DependencyResolution.CompositeResolver2.GetService(Type type, Object key)
in System.Data.Entity.Infrastructure.DependencyResolution.DbDependencyResolverExtensions.GetService[T](IDbDependencyResolver resolver, Object key)
in System.Data.Entity.Internal.LazyInternalConnection.CreateConnectionFromProviderName(String providerInvariantName)
in System.Data.Entity.Internal.LazyInternalConnection.InitializeFromConnectionStringSetting(ConnectionStringSettings appConfigConnection)
in System.Data.Entity.Internal.LazyInternalConnection.TryInitializeFromAppConfig(String name, AppConfig config)
in System.Data.Entity.Internal.LazyInternalConnection.Initialize()
in System.Data.Entity.Internal.LazyInternalConnection.get_Connection()
in System.Data.Entity.Internal.LazyInternalContext.get_Connection()
in System.Data.Entity.Infrastructure.DbContextInfo..ctor(Type contextType, DbProviderInfo modelProviderInfo, AppConfig config, DbConnectionInfo connectionInfo, Func1 resolver)
in System.Data.Entity.Migrations.DbMigrator..ctor(DbMigrationsConfiguration configuration, DbContext usersContext, DatabaseExistenceState existenceState, Boolean calledByCreateDatabase)
in System.Data.Entity.Migrations.DbMigrator..ctor(DbMigrationsConfiguration configuration)
in System.Data.Entity.Migrations.Design.MigrationScaffolder..ctor(DbMigrationsConfiguration migrationsConfiguration)
in System.Data.Entity.Migrations.Design.ToolingFacade.ScaffoldRunner.Run()
in System.AppDomain.DoCallBack(CrossAppDomainDelegate callBackDelegate)
in System.AppDomain.DoCallBack(CrossAppDomainDelegate callBackDelegate)
in System.Data.Entity.Migrations.Design.ToolingFacade.Run(BaseRunner runner)
in System.Data.Entity.Migrations.Design.ToolingFacade.ScaffoldInitialCreate(String language, String rootNamespace)
in System.Data.Entity.Migrations.EnableMigrationsCommand.<>c__DisplayClass2.<.ctor>b__0()
in System.Data.Entity.Migrations.MigrationsDomainCommand.Execute(Action command)

Failed to find or load the registered .Net Framework Data Provider.

Can you help me?

Thanks in advance.