Cannot GET / error in angular 6 .net core 2.1

I want to show toast notification in my angular form. I am using visual studio .net core 2.1 template...I am getting

Cannot GET / error when running localhost...

running ng build following errors shown...

../../../../../node_modules/ng2-toasty/index.d.ts(1,37): error TS2307: Cannot find module '@angular/core'. ../../../../../node_modules/ng2-toasty/src/shared.d.ts(1,30): error TS2307: Cannot find module '@angular/platform-browser'. ../../../../../node_modules/ng2-toasty/src/shared.d.ts(2,31): error TS2307: Cannot find module '@angular/core'. ../../../../../node_modules/ng2-toasty/src/toast.component.d.ts(1,30): error TS2307: Cannot find module '@angular/core'. ../../../../../node_modules/ng2-toasty/src/toasty.component.d.ts(1,24): error TS2307: Cannot find module '@angular/core'. ../../../../../node_modules/ng2-toasty/src/toasty.service.d.ts(1,28): error TS2307: Cannot find module 'rxjs/Observable'.

AppModule:

import { VehicleFormComponent } from './vehicle-form/vehicle-form.component';
import { VehicleService } from './services/vehicle.service';
import { ToastyModule } from 'ng2-toasty';
import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';
import { FormsModule } from '@angular/forms';
import { HttpClientModule } from '@angular/common/http';
import { RouterModule } from '@angular/router';

@NgModule({
  declarations: [
    AppComponent,
    NavMenuComponent,
    HomeComponent,
    VehicleFormComponent
  ],
  imports: [
    BrowserModule.withServerTransition({
      appId: 'ng-cli-universal'
    }),
    FormsModule,
    HttpClientModule,
    FormsModule,
    ToastyModule.forRoot(),
    RouterModule.forRoot([
      {
        path: '',
        component: HomeComponent,
        pathMatch: 'full'
      },
      {
        path: 'vehicles/new',
        component: VehicleFormComponent
      }
    ])
  ],
  providers: [VehicleService],
  bootstrap: [AppComponent]
})
export class AppModule {}