The project is based on Angular 13.

Added routing to the project. app-routing.module.ts

import { NgModule } from '@angular/core';
import {RouterModule, Routes} from "@angular/router";
import {HomePageComponent} from "./home-page/home-page.component";

const appRoutes: Routes =[
  { path: '', component: HomePageComponent}
];

@NgModule({
  imports: [RouterModule.forRoot(appRoutes)],
  exports: [RouterModule]
})
export class AppRoutingModule { }

added AppRoutingModule to the file app.module.ts

import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { AppComponent } from './app.component';
import {ModalModule} from "ngb-modal";
import {FormsModule} from "@angular/forms";
import { HomePageComponent } from './home-page/home-page.component';
import {AppRoutingModule} from "./app-routing.module";

@NgModule({
  imports: [
    BrowserModule,
    FormsModule,
    AppRoutingModule
  ],
  declarations: [
    AppComponent,
    HomePageComponent
  ],

  providers: [],
  bootstrap: [AppComponent]
})
export class AppModule { }

but if I try to add to the file app.component.html

<router-outlet></router-outlet>

IDEA throws an error

Cannot resolve symbol 'router-outlet'

But if I open another project with routing in the same IDE, there is no such error.