You need to pass array with SQL condition
$invoice = Invoices::findFirst([
'id_user = ?0 AND id_menu = ?1',
'bind' => [$iduser, $idmenu],
]);
or
$invoice = Invoices::findFirst([
'id_user = :iduser: AND id_menu = :idmenu:',
'bind' => ['iduser' => $iduser, 'idmenu' => $idmenu],
]);
More examples: https://docs.phalcon.io/4.0/en/db-models#findfirst