I am using ASP.NET Core MVC 3.0 API project. I am using `Microsoft.AspNetCore.Identity` v3.0

In the `Startup.cs`, inside the `ConfigureServices` method, when I type the below and run the application, I get an exception:

```
var builder = services.AddIdentityCore<AppUser>();
var identityBuilder = new IdentityBuilder(builder.UserType, builder.Services);
identityBuilder.AddEntityFrameworkStores<DataContext>();
identityBuilder.AddSignInManager<SignInManager<AppUser>>();

```

The `AppUser` class is something like this:

```
using Microsoft.AspNetCore.Identity;

namespace Domain
{
    public class AppUser : IdentityUser
    {
        public string DisplayName { get; set; }
    }
}

```

The exception I am getting is this:

> 
> Unhandled exception. System.AggregateException: Some services are not able to be constructed (Error while validating the service descriptor 'ServiceType: Microsoft.AspNetCore.Identity.ISecurityStampValidator Lifetime: Scoped ImplementationType: Microsoft.AspNetCore.Identity.SecurityStampValidator`1[Domain.AppUser]': Unable to resolve service for type 'Microsoft.AspNetCore.Authentication.ISystemClock' while attempting to activate 'Microsoft.AspNetCore.Identity.SecurityStampValidator`1[Domain.AppUser]'.) (Error while validating the service descriptor 'ServiceType: Microsoft.AspNetCore.Identity.ITwoFactorSecurityStampValidator Lifetime: Scoped ImplementationType: Microsoft.AspNetCore.Identity.TwoFactorSecurityStampValidator`1[Domain.AppUser]': Unable to resolve service for type 'Microsoft.AspNetCore.Authentication.ISystemClock' while attempting to activate 'Microsoft.AspNetCore.Identity.TwoFactorSecurityStampValidator`1[Domain.AppUser]'.)
> 
> 
> System.InvalidOperationException: Error while validating the service descriptor 'ServiceType: Microsoft.AspNetCore.Identity.ISecurityStampValidator Lifetime: Scoped ImplementationType: Microsoft.AspNetCore.Identity.SecurityStampValidator`1[Domain.AppUser]': Unable to resolve service for type 'Microsoft.AspNetCore.Authentication.ISystemClock' while attempting to activate 'Microsoft.AspNetCore.Identity.SecurityStampValidator`1[Domain.AppUser]'.
> 
> 
> System.InvalidOperationException: Unable to resolve service for type 'Microsoft.AspNetCore.Authentication.ISystemClock' while attempting to activate 'Microsoft.AspNetCore.Identity.SecurityStampValidator`1[Domain.AppUser]'.
> 
> 
> at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteFactory.CreateArgumentCallSites(Type serviceType, Type implementationType, CallSiteChain callSiteChain, ParameterInfo[] parameters, Boolean throwIfCallSiteNotFound)  
> 
>  at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteFactory.CreateConstructorCallSite(ResultCache lifetime, Type serviceType, Type implementationType, CallSiteChain callSiteChain)  
> 
>  at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteFactory.TryCreateExact(ServiceDescriptor descriptor, Type serviceType, CallSiteChain callSiteChain, Int32 slot)  
> 
>  at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteFactory.GetCallSite(ServiceDescriptor serviceDescriptor, CallSiteChain callSiteChain)  
> 
>  at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngine.ValidateService(ServiceDescriptor descriptor)  
> 
>  --- End of inner exception stack trace ---  
> 
>  at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngine.ValidateService(ServiceDescriptor descriptor)  
> 
>  at Microsoft.Extensions.DependencyInjection.ServiceProvider..ctor(IEnumerable`1 serviceDescriptors, ServiceProviderOptions options)  
> 
>  --- End of inner exception stack trace ---  
> 
>  at Microsoft.Extensions.DependencyInjection.ServiceProvider..ctor(IEnumerable`1 serviceDescriptors, ServiceProviderOptions options)
>  at Microsoft.Extensions.DependencyInjection.ServiceCollectionContainerBuilderExtensions.BuildServiceProvider(IServiceCollection services, ServiceProviderOptions options)
>  at Microsoft.Extensions.DependencyInjection.DefaultServiceProviderFactory.CreateServiceProvider(IServiceCollection containerBuilder)
>  at Microsoft.Extensions.Hosting.Internal.ServiceFactoryAdapter`1.CreateServiceProvider(Object containerBuilder)
>  at Microsoft.Extensions.Hosting.HostBuilder.CreateServiceProvider()
>  at Microsoft.Extensions.Hosting.HostBuilder.Build()
>  at Api.Program.Main(String[] args) in C:\Projects\XXXXXXXX\api\Program.cs:line 19
>  ---> (Inner Exception #1) System.InvalidOperationException: Error while validating the service descriptor 'ServiceType: Microsoft.AspNetCore.Identity.ITwoFactorSecurityStampValidator Lifetime: Scoped 
>  ImplementationType: Microsoft.AspNetCore.Identity.TwoFactorSecurityStampValidator`1[Domain.AppUser]': Unable to resolve service for type 'Microsoft.AspNetCore.Authentication.ISystemClock' while attempting to activate 'Microsoft.AspNetCore.Identity.TwoFactorSecurityStampValidator`1[Domain.AppUser]'.
>  ---> System.InvalidOperationException: Unable to resolve service for type 'Microsoft.AspNetCore.Authentication.ISystemClock' while attempting to activate 'Microsoft.AspNetCore.Identity.TwoFactorSecurityStampValidator`1[Domain.AppUser]'.
>  at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteFactory.CreateArgumentCallSites(Type serviceType, Type implementationType, CallSiteChain callSiteChain, ParameterInfo[] parameters, 
>  Boolean throwIfCallSiteNotFound)
>  at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteFactory.CreateConstructorCallSite(ResultCache lifetime, Type serviceType, Type implementationType, CallSiteChain callSiteChain)  
> 
>  at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteFactory.TryCreateExact(ServiceDescriptor descriptor, Type serviceType, CallSiteChain callSiteChain, Int32 slot)
>  at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteFactory.GetCallSite(ServiceDescriptor serviceDescriptor, CallSiteChain callSiteChain)
>  at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngine.ValidateService(ServiceDescriptor descriptor)
>  --- End of inner exception stack trace ---
>  at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngine.ValidateService(ServiceDescriptor descriptor)
>  at Microsoft.Extensions.DependencyInjection.ServiceProvider..ctor(IEnumerable`1 serviceDescriptors, ServiceProviderOptions options)<---
> 
> 
> 

Any idea what I am doing wrong?