'ion-header' is not a known element ionic 5

Question

Following are the steps i have taken

1. I created a new project of ionic 5 using ionic start template blank
2. updated angular to angular 9
3. created a new page module using ng g page main
4. listed in AppRoutingModule using lazyloading.

I am getting following error

ERROR in src/app/pages/main/main.page.html:1:1 - error NG8001: 'ion-header' is not a known element:
1. If 'ion-header' is an Angular component, then verify that it is part of this module.
2. If 'ion-header' is a Web Component then add 'CUSTOM_ELEMENTS_SCHEMA' to the '@NgModule.schemas' of this component to suppress this message.

1 <ion-header>
  ~~~~~~~~~~~~

  src/app/pages/main/main.page.ts:5:16
    5   templateUrl: './main.page.html',
                     ~~~~~~~~~~~~~~~~~~
    Error occurs in the template of component MainPage.
src/app/pages/main/main.page.html:2:3 - error NG8001: 'ion-toolbar' is not a known element:
1. If 'ion-toolbar' is an Angular component, then verify that it is part of this module.
2. If 'ion-toolbar' is a Web Component then add 'CUSTOM_ELEMENTS_SCHEMA' to the '@NgModule.schemas' of this component to suppress
this message.

2   <ion-toolbar>
    ~~~~~~~~~~~~~

  src/app/pages/main/main.page.ts:5:16
    5   templateUrl: './main.page.html',
                     ~~~~~~~~~~~~~~~~~~
    Error occurs in the template of component MainPage.
src/app/pages/main/main.page.html:3:5 - error NG8001: 'ion-title' is not a known element:
1. If 'ion-title' is an Angular component, then verify that it is part of this module.
2. If 'ion-title' is a Web Component then add 'CUSTOM_ELEMENTS_SCHEMA' to the '@NgModule.schemas' of this component to suppress this message.

3     <ion-title>main</ion-title>
      ~~~~~~~~~~~

  src/app/pages/main/main.page.ts:5:16
    5   templateUrl: './main.page.html',
                     ~~~~~~~~~~~~~~~~~~
    Error occurs in the template of component MainPage.
src/app/pages/main/main.page.html:7:1 - error NG8001: 'ion-content' is not a known element:
1. If 'ion-content' is an Angular component, then verify that it is part of this module.
2. If 'ion-content' is a Web Component then add 'CUSTOM_ELEMENTS_SCHEMA' to the '@NgModule.schemas' of this component to suppress
this message.

7 <ion-content>
  ~~~~~~~~~~~~~

I am missing something very obvious.

Following is the code

app.module.ts

import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { RouteReuseStrategy } from '@angular/router';

import { IonicModule, IonicRouteStrategy } from '@ionic/angular';
import { SplashScreen } from '@ionic-native/splash-screen/ngx';
import { StatusBar } from '@ionic-native/status-bar/ngx';

import { AppComponent } from './app.component';
import { AppRoutingModule } from './app-routing.module';
import { BrowserAnimationsModule } from '@angular/platform-browser/animations';

@NgModule({
  declarations: [AppComponent],
  entryComponents: [],
  imports: [BrowserModule, IonicModule.forRoot(), AppRoutingModule, BrowserAnimationsModule],
  providers: [
    StatusBar,
    SplashScreen,
    { provide: RouteReuseStrategy, useClass: IonicRouteStrategy }
  ],
  bootstrap: [AppComponent]
})
export class AppModule {}

app-routing.module.ts

import { NgModule } from '@angular/core';
import { PreloadAllModules, RouterModule, Routes } from '@angular/router';

const routes: Routes = [
  {
    path: '',
    loadChildren: () => import('./pages/main/main-routing.module').then(m => m.MainPageRoutingModule)
  },
];

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

main.module.ts

import { NgModule } from '@angular/core';
import { CommonModule } from '@angular/common';
import { FormsModule } from '@angular/forms';

import { IonicModule } from '@ionic/angular';

import { MainPageRoutingModule } from './main-routing.module';

import { MainPage } from './main.page';

@NgModule({
  imports: [
    CommonModule,
    FormsModule,
    IonicModule,
    MainPageRoutingModule
  ],
  declarations: [MainPage]
})
export class MainPageModule {}

main.page.ts

import { Component, OnInit } from '@angular/core';

@Component({
  selector: 'sxm-main',
  templateUrl: './main.page.html',
  styleUrls: ['./main.page.scss'],
})
export class MainPage implements OnInit {

  constructor() { }

  ngOnInit() {
  }

}

main.page.html

<ion-header>
  <ion-toolbar>
    <ion-title>main</ion-title>
  </ion-toolbar>
</ion-header>

<ion-content>

</ion-content>

Answer 1

In my case I was getting weird html template errors including the one OP describes because I created new component and forgot to add it to the module declarations

app.module.ts

@NgModule({
...
declarations: [MyComponent]
}

Answer 2

I had the same issue and I simply forgot to add the IonicModule to the new module I created that contained the components throwing the exceptions.

Check to see if all your modules import IonicModule

import { NgModule } from '@angular/core';
import { CommonModule } from '@angular/common';

import { IonicModule } from '@ionic/angular';

import { ExamplePage } from './contactus.page';

@NgModule({
  imports: [
    CommonModule,
    IonicModule
  ],
  declarations: [ExamplePage]
})
export class ExampleModule {}