error NG8002: Can't bind to 'appDropdown' since it isn't a known property of 'button'. code example
Example: Uncaught Error: Template parse errors: Can't bind to 'ngModel'
Try to import FormModule an app.module.ts file
import { FormsModule } from '@angular/forms'
@NgModule({
declarations: [
....
],
imports: [
...,
FormsModule
],
providers: [],
bootstrap: [AppComponent]
})
export class AppModule { }