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

Example: 'mat-label' is not a known element:

// common-widgets.module.ts
import { MatFormFieldModule } from '@angular/material/form-field';
import { MatInputModule } from '@angular/material/input';

@NgModule({
  imports: [
    MatFormFieldModule,
    MatInputModule
  ]
})
export class CommonWidgetsModule { }