'router-outlet' is not a known element in angular 6

You are getting the error because you are using the router-outlet component in the UIModule and didn't import the RouterModule inside UIModule.

I see you are missing RouterModule inside the imports of UIModule

@NgModule({
  imports: [
    CommonModule,
    RouterModule
  ],
  declarations: [LayoutComponent, HeaderComponent, FooterComponent],
  exports: [LayoutComponent]
})
export class UiModule { }