As always, we need to put our new component's references within the app.module.shared.ts file. A good place for them would be just below the LoginComponent ones:

[...]

import { LoginComponent } from './components/login/login.component';
import { RegisterComponent } from './components/user/register.component';

[...]

declarations: [
    [...]
    LoginComponent,
    RegisterComponent,

[...]

RouterModule.forRoot([
    { path: 'login', component: LoginComponent },
    { path: 'register', component: RegisterComponent },

[...]