f 'mat-error' is a Web Component then add 'CUSTOM_ELEMENTS_SCHEMA' to the '@NgModule.schemas' of this component to suppress this message. code example

Example 1: 'mat-form-field' is not a known element:

@NgModule({
  imports: [
    MatButtonModule,
    MatFormFieldModule,
    MatInputModule,
    MatRippleModule,
 ]
 ....

Example 2: 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';