after upgrading to angular 12, all material components are "not a known element"

After upgrading to Angular 12, although I have all the required code in app.module.ts, all the material tags are not recognized: for example, for mat-checkbox, I have the following in app.module.ts

import { MatCheckboxModule } from '@angular/material/checkbox';

@NgModule({
imports: [
BrowserModule,
MatCheckboxModule,


and at execution:

'mat-checkbox' is not a known element:

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

And same problem for all the material tags (mat-icon, mat-table, mat-label . . .).

In previous version (Angular 9), all was OK. Thanks for help.

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