AngularFIRE Property 'subscribe' does not exist on type 'AngularFireList<{}>'

export class AppComponent {
  constructor(db : AngularFireDatabase){

In Angular firebase version 5.0 and above .subscribe is available after .valuechanges()

A simpler change would to be add valueChanges() before .subscribe()

db.list('/Country/countries').valueChanges().subscribe(countries => {
    this.countries = countries;

Starting in AngularFire 5.0 you'll want to use one of snapshotChanges(), valueChanges<T>(), stateChanges(), or auditTrail(). See the 5.0 migration guide.

Get started with the most basic, valueChanges():

export class AppComponent {
  countries: Observable<Country[]>;
  constructor(db: AngularFireDatabase ) {
    this.countries = db.list('/Country/countries').valueChanges();