error NG6002: Appears in the NgModule.imports of AppModule, but could not be resolved to an NgModule class

Your module is not yet loaded by the Angular Server in node ng serve, so restart your server so the server loads the module that you just added in @NgModule app.module.ts

This error shows when you add component declaration in imports: [] instead of declarations: [], e.g:

declarations: [
  AppComponent,
],
imports: [
  BrowserModule,
  AppRoutingModule,
  SomeComponent <-----------wrong
],

I got this error when I made the bonehead mistake of importing MatSnackBar instead of MatSnackBarModule in app.module.ts.