make sure you have imported the correct Validate class use Livewire\Attributes\Validate;
this should work then:
#[Validate('unique:users,email')]
public $email;
but bear in mind, you cant put more complex rules in there (like ignoring the current user in the unique rule). from the documentation: https://livewire.laravel.com/docs/validation#validate-attributes
Validate attributes don't support Rule objects PHP Attributes are restricted to certain syntaxes like plain strings and arrays. If you find yourself wanting to use run-time syntaxes like Laravel's Rule objects (Rule::exists(...)) you should instead define a rules() method in your component.