总结：
您当前的问题404是由于注解掉路由模板而导致的。但是如果您使用路由模板，您将遇到一个新的问题，即找不到方法，因为您的Web项目是.net core3.1，它使用带有包Microsoft.AspNetCore.Identity 3.1.0的SigninManager，而您的Services项目是.netstandard 2.0，它将SigninManager与Microsoft.AspNetCore.Identity 2.2.0一起使用。这将导致找不到该方法。两个不同版本的包使用两个不同的SigninManager构造函数。Microsoft.AspNetCore.Identity 2.2.0中的登录管理器：

public class SignInManager<TUser> where TUser : class
{ 
    public SignInManager(UserManager<TUser> userManager, IHttpContextAccessor contextAccessor, IUserClaimsPrincipalFactory<TUser> claimsFactory, IOptions<IdentityOptions> optionsAccessor, ILogger<SignInManager<TUser>> logger, IAuthenticationSchemeProvider schemes);

Microsoft.AspNetCore.Identity 3.1.0中的登录管理器：

public class SignInManager<TUser> where TUser : class
{ 
    public SignInManager(UserManager<TUser> userManager, IHttpContextAccessor contextAccessor, IUserClaimsPrincipalFactory<TUser> claimsFactory, IOptions<IdentityOptions> optionsAccessor, ILogger<SignInManager<TUser>> logger, IAuthenticationSchemeProvider schemes,
                                      IUserConfirmation<TUser> confirmation);

在MovieDatabase.Web项目中，您应该使用下面的路线模板：

app.UseEndpoints(endpoint =>
{
    endpoint.MapControllerRoute("admin", "{area:exists}/{controller=Dashboard}/{action=Index}/{id?}");
    endpoint.MapControllerRoute(name: "default", "{controller=Home}/{action=Index}/{id?}");
    endpoint.MapRazorPages();
});

在MovieDatabase.Services项目中，更改您的项目文件和ApplicationSignInManager.cs如下：
1.将TargetFramework更改为netcoreapp3.1，并将Microsoft.AspNetCore.App的FrameworkReference相加：

<PropertyGroup>
    <TargetFramework>netcoreapp3.1</TargetFramework>
    <LangVersion>latest</LangVersion>
</PropertyGroup>
<ItemGroup>
 
  <FrameworkReference Include="Microsoft.AspNetCore.App" />

  <PackageReference Include="RestSharp" Version="106.6.2" />
  <PackageReference Include="StyleCop.Analyzers" Version="1.1.0-beta009" PrivateAssets="all">
    <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
  </PackageReference>
</ItemGroup>

2：将ApplicationSignInManager.cs：

public class ApplicationSignInManager<TUser> : SignInManager<ApplicationUser>
    where TUser : ApplicationUser
{
    public ApplicationSignInManager(
        ApplicationUserManager<ApplicationUser> userManager,
        IHttpContextAccessor contextAccessor,
        IUserClaimsPrincipalFactory<ApplicationUser> claimsFactory,
        IOptions<IdentityOptions> optionsAccessor,
        ILogger<SignInManager<ApplicationUser>> logger,
        IAuthenticationSchemeProvider schemeProvider,
        IUserConfirmation<ApplicationUser> userConfirmation       //add this...
    )
    : base(userManager, contextAccessor, claimsFactory, optionsAccessor, logger, schemeProvider, userConfirmation)
    {
    }