'ng-select' is not a known element

You have to import the NgSelectModule in the module that declares your ModalTestComponent, or, if your ComponentsModule is a wrapper, you should import the module that declares ModalTestComponent in it; this way:

import { NgModule } from '@angular/core';
import { CommonModule } from '@angular/common';
import { HttpClientModule } from '@angular/common/http';

import { FormsModule, ReactiveFormsModule } from '@angular/forms';
import { NgSelectModule } from '@ng-select/ng-select';
// add this line 
import { ModuleThatDeclaresModalTest } from 'path/to/module'


@NgModule({
  imports: [
    CommonModule,
    HttpClientModule,
    FormsModule,
    ReactiveFormsModule,
    NgSelectModule,
    // and this line 
    ModuleThatDeclaresModalTest 
   ]

})

export class ComponentsModule { }

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As an alternative, as @Narm said, you can import NgSelectModule in your AppModule, so that all children modules can use it.