此问题可能是因为未正确配置服务集合。确保在Startup.cs文件中正确地注入了Identity服务并配置了Identity选项。

以下是正确注册Identity服务和配置Identity选项的示例代码：

public void ConfigureServices(IServiceCollection services)
{
    // 注册 Identity 服务
    services.AddDefaultIdentity(options =>
    {
        // 配置 Password 验证规则
        options.Password.RequireDigit = true;
        options.Password.RequireLowercase = true;
        options.Password.RequireNonAlphanumeric = true;
        options.Password.RequireUppercase = true;
        options.Password.RequiredLength = 8;
    })
    .AddRoles()
    .AddEntityFrameworkStores();

    // 添加认证服务
    services.AddAuthentication(options =>
    {
        options.DefaultScheme = IdentityConstants.ApplicationScheme;
        options.DefaultSignInScheme = IdentityConstants.ExternalScheme;
    })
    .AddIdentityCookies();

    // 添加授权服务
    services.AddAuthorization(options =>
    {
        options.AddPolicy("RequireAdministratorRole", policy => policy.RequireRole("Administrator"));
    });
}

确保在使用UserManager和RoleManager时注入了正确的服务（例如，ApplicationUser和IdentityRole）。

以下是在Blazor页面中使用UserManager和RoleManager的示例代码：

@inject UserManager UserManager
@inject RoleManager RoleManager

@code {
    private async Task CreateUser()
    {
        var user = new ApplicationUser { UserName = "testuser@example.com", Email = "testuser@example.com" };
        var result = await UserManager.CreateAsync(user, "MyP@ssw0rd!");
        if (result.Succeeded)
        {
            await UserManager.AddToRoleAsync(user, "User");
        }
    }

    private async Task CreateRole()
    {
        var role = new IdentityRole { Name = "Administrator" };
        var result = await RoleManager.CreateAsync(role);
    }
}