'router-outlet' is not a known element

If you getting this type of error in your's angular project when you configure your router module file.

then just check the routing module file and check the import and export array.

you must be declare the RouterModule in your's export array.

In my case app.routing module file 

@NgModule({

imports: [

RouterModule.forRoot(appRoutes),

],

exports: [

RouterModule

]