How to use mat-tab? mat-tab is not a known element error

Add the import in your module.ts file and add it to imports (if you use several module.ts files, add it in the one which is responsible for your component).

import { MatTabsModule } from '@angular/material';

@NgModule({
  imports: [
    ...
    MatTabsModule,
    ...
  ],
  declarations: [
    ...
  ],
  providers: []
})

export class AppModule {}