Property 'alert' does not exist on type 'LoginPage'

Hi, I,m using ionic 4 to build a mobile apps. I got an error when I try to connect login and signup with firebase. But I got error Property ‘alert’ does not exist on type ‘LoginPage’ at line const alert = await this.alert.create. Can anyone help me with this?

import { Component, ViewEncapsulation } from '@angular/core';
import { NgForm } from '@angular/forms';
import { Router } from '@angular/router';
import { AngularFireAuth } from '@angular/fire/auth'
import { auth } from 'firebase/app'
import{ AlertController } from '@ionic/angular'
import { UserData } from '../../providers/user-data';

import { UserOptions } from '../../interfaces/user-options';



@Component({
  selector: 'page-login',
  templateUrl: 'login.html',
  styleUrls: ['./login.scss'],
})
export class LoginPage {
  login: UserOptions = { username: '', password: '', copassword:'' };
  submitted = false;

  constructor(
    public afAuth: AngularFireAuth,
    public userData: UserData,
    public router: Router,
    
  ) { }

  async onlogin (username: string, password: string) {
    this.submitted = true;
    
    try{

      const res = await this.afAuth.auth.signInWithEmailAndPassword(username +'@codedamn.com',password)
      console.log(res)
      this.router.navigate(['/tabs'])
    } catch(error){
      console.dir(error)
      this.showAlert("Error", "User not found")

      if (error.code =="auth/user-not-found"){
        console.log("User not found")
      }
    }
  }
  async showAlert(header: string, message: string) {
    const alert = await this.alert.create ({ // error is here
      header,
      message,
      buttons: ["Ok"]
    })

    await alert.present()
  }
}

solved by adding public alert: AlertController