1. If 'mat-slider' is an Angular component, then verify that it is part of this module.2. If 'mat-slider' is a Web Component then add 'CUSTOM_ELEMENTS_SCHEMA' to the '@NgModule.schemas' of this component to suppress this message. code example

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

@NgModule({
  imports: [
  	...
	MatFormFieldModule,
    MatInputModule,
    ...
  ]
})