Error 404 Url Not Found when using InMemoryDbService in Angular

I'm doing a web application in Angular 8.3.21. I'm trying to implemente a fake backend using InMemoryDbService to test my http requests. I'm also using faker to create random id's and words.

I have installed this:

npm install --save angular-in-memory-web-api
npm i faker --save
npm install @types/faker --save

My module:

@NgModule({
  declarations: [
    // A few components...
  ],
  imports: [
    HttpClientModule
    environment.production ? // "forFeature" because this is a module of a feature 
                                of my app which is loaded lazily
      [] : HttpClientInMemoryWebApiModule.forFeature(
        AppointmentFakeBackendService
      ),
  ],
  providers: [
    AppointmentService,
    AppointmentFakeBackendService
  ]
})

The fake backend:

import {Injectable} from '@angular/core';
import {InMemoryDbService} from 'angular-in-memory-web-api';
import * as faker from 'faker/locale/es';
import {Observable} from 'rxjs';

@Injectable()
export class AppointmentFakeBackendService implements InMemoryDbService {

  constructor() {
  }

  createDb(): {} | Observable<{}> | Promise<{}> {
    const administrations = [
      {id: faker.random.uuid(), administration: faker.commerce.department()},
    ];

    return {administrations};
  }
}

My service:

import {Injectable} from '@angular/core';
import {HttpClient} from '@angular/common/http';
import {Observable} from 'rxjs';
import {tap} from 'rxjs/operators';

@Injectable()
export class AppointmentService {

  private url = 'api/administrations';

  constructor(
    private http: HttpClient
  ) {
  }

  getAdministrations(): Observable<any> {
    return this.http.get<any>(this.url).pipe(
      tap(data => console.log(data))
    );
  }
}

Finally, in my component I call the method from the service to get the data:

constructor(
    private service: AppointmentService
  ) {
  }

  ngOnInit() {
    this.service.getAdministrations().subscribe(res => {
      console.log(res);
    });
  }

Errors:

GET http://localhost:4200/api/administrations 404 (Not Found)

What I'm missing? I have followed the official examples and I have tested solutions out there, but, didn't work for me.

I have changed this in the fake backend without success:

createDb(): {} | Observable<{}> | Promise<{}> {
    const data = [
      {id: faker.random.uuid(), administration: faker.commerce.department()},
    ];

    return {administrations: data}; // <== This line
  }

My goal is to create a fake backend, generating random information using faker and use a service to get the data in my component.