If 'main-nav' is a Web Component then add 'CUSTOM_ELEMENTS_SCHEMA' to the '@NgModule.schemas' of this component to suppress this message. code example

Example: 2. If 'app-navbar' is a Web Component then add 'CUSTOM_ELEMENTS_SCHEMA' to the '@NgModule.schemas' of this component to suppress this message.

import { NgModule, CUSTOM_ELEMENTS_SCHEMA } from '@angular/core';