firebase_app__WEBPACK_IMPORTED_MODULE_5__.app(...).functions is not a function

"@angular/core": "~9.1.2",
"@angular/fire": "^5.1.1",
"firebase": "^7.24.0",

createPortalLink() is working js code imported to angular, but it generates the TypeError. How do I fix the TypeError?

TypeError: firebase_app__WEBPACK_IMPORTED_MODULE_5__.app(...).functions is not a function

import * as firebase from 'firebase/app';

async createPortalLink() {
  // Call billing portal function
    const functionLocation = 'us-central1';
    const functionRef = firebase
      .app()
      .functions(functionLocation)
      .httpsCallable('ext-firestore-stripe-subscriptions-createPortalLink');
    const { data } = await functionRef({ returnUrl: window.location.origin });
    window.location.assign(data.url);
  }

core.js:6210 ERROR Error: Uncaught (in promise): TypeError: firebase_app__WEBPACK_IMPORTED_MODULE_5__.app(...).functions is not a function
TypeError: firebase_app__WEBPACK_IMPORTED_MODULE_5__.app(...).functions is not a function
    at BillingService.<anonymous> (billing.service.ts:52)
    at Generator.next (<anonymous>)
    at tslib.es6.js:73
    at new ZoneAwarePromise (zone-evergreen.js:960)
    at __awaiter (tslib.es6.js:69)
    at BillingService.callBillingPortalFunction (billing.service.ts:47)
    at BillingComponent.<anonymous> (billing.component.ts:77)
    at Generator.next (<anonymous>)
    at tslib.es6.js:73
    at new ZoneAwarePromise (zone-evergreen.js:960)
    at resolvePromise (zone-evergreen.js:798)
    at zone-evergreen.js:705
    at rejected (tslib.es6.js:71)
    at ZoneDelegate.invoke (zone-evergreen.js:364)
    at Object.onInvoke (core.js:41731)
    at ZoneDelegate.invoke (zone-evergreen.js:363)
    at Zone.run (zone-evergreen.js:123)
    at zone-evergreen.js:857
    at ZoneDelegate.invokeTask (zone-evergreen.js:399)
    at Object.onInvokeTask (core.js:41709)

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