No sign-out authentication handler is registered for the scheme 'Identity.TwoFactorUserId'

As a complement to @Luke's answer:

The reason why SignInManager::SignOutAsync() throws is this method will also sign out the TwoFactorUserIdScheme behind the scenes:

public virtual async Task SignOutAsync()
{
    await Context.SignOutAsync(IdentityConstants.ApplicationScheme);
    await Context.SignOutAsync(IdentityConstants.ExternalScheme);
    await Context.SignOutAsync(IdentityConstants.TwoFactorUserIdScheme);
}

(See source code)

Typically, these tree authentication schemes are registered automatically by AddIdentity<TUser, TRole>():

public static IdentityBuilder AddIdentity<TUser, TRole>(
    this IServiceCollection services,
    Action<IdentityOptions> setupAction)
{
    services.AddAuthentication(options =>
    {
        options.DefaultAuthenticateScheme = IdentityConstants.ApplicationScheme;
        options.DefaultChallengeScheme = IdentityConstants.ApplicationScheme;
        options.DefaultSignInScheme = IdentityConstants.ExternalScheme;
    })
    ...
    .AddCookie(IdentityConstants.TwoFactorUserIdScheme, o =>
    {
        o.Cookie.Name = IdentityConstants.TwoFactorUserIdScheme;
        o.ExpireTimeSpan = TimeSpan.FromMinutes(5);
    });
    ... // other services
}

(See source code )

However, you added the Identity services by AddIdentityCore<>() instead of the AddIdentity<>().

Because the AddIdentityCore<>() doesn't register a TwoFactorUserIdScheme scheme (see source code) automatically, there's no associated CookieAuthenticationHandler for TwoFactorUserIdScheme. As a result, it throws.

How to solve

In order to work with SignInManager.SignOutAsync(), according to above description, we need ensure a <scheme>-<handler> map has been registed for TwoFactorUserIdScheme .

So I change your code as below, now it works fine for me:

serviceCollection.AddAuthentication(IdentityConstants.ApplicationScheme)
    .AddCookie(IdentityConstants.ApplicationScheme, options =>
    {
        options.SlidingExpiration = true;
    })
    .AddCookie(IdentityConstants.TwoFactorUserIdScheme, o =>
    {
        o.Cookie.Name = IdentityConstants.TwoFactorUserIdScheme;
        o.ExpireTimeSpan = TimeSpan.FromMinutes(5);
    })
    .AddGoogle(googleOptions =>
    {
        this.Configuration.Bind("OAuth2:Providers:Google", googleOptions);
        googleOptions.ClaimActions.MapJsonKey(ClaimTypes.NameIdentifier, "sub", "string");
    })
    .AddExternalCookie();

Also you can create your own SignInManager<MyUser> and override sign out as you need

public class CustomSignInManager : SignInManager<MyUser>
{
    public override async Task SignOutAsync()
    {    
        await Context.SignOutAsync(IdentityConstants.ApplicationScheme);
        await Context.SignOutAsync(GoogleDefaults.AuthenticationScheme);
    }
}

Then change AddSignInManager<SignInManager<MyUser>>() to AddSignInManager<CustomSignInManager>() in your Startup class

serviceCollection
    .AddIdentityCore<MyUser>(identityOptions =>
    {
        identityOptions.SignIn.RequireConfirmedEmail = false;
    })
    .AddUserStore<MyUserStore>()
    .AddSignInManager<CustomSignInManager>();

Do not use the SignOutAsync method on a SignInManager<T> you've injected into the controller. Instead, use the method on the HttpContext which takes a scheme argument. I don't know why.

Below code works for me , use the same AuthenticationScheme that you use while "AddAuthentication" in startup.cs

[HttpGet("signout")]
[AllowAnonymous]
public async Task signout()
{
    await HttpContext.SignOutAsync(CookieAuthenticationDefaults.AuthenticationScheme);

    var prop = new AuthenticationProperties
    {
        RedirectUri = "/logout-complete"
    };
    // after signout this will redirect to your provided target
    await HttpContext.SignOutAsync(OpenIdConnectDefaults.AuthenticationScheme, prop);
}

[HttpGet("logout-complete")]
[AllowAnonymous]
public string logoutComplete()
{
    return "logout-complete";
}