The page actually exists https://fr.wikisource.org/wiki/Balzac,_sa_m%C3%A9thode_de_travail. In any case, OPDS command should probably log the failure and continue.
Error
00:03:50 CRITICAL [console] Error thrown while running command "app:opds -l fr --category=Bon_pour_export". Message: "Page not found for: Balzac,_sa_méthode_de_travail" ["exception" => Symfony\Component\HttpKernel\Exception\NotFoundHttpException { …},"command" => "app:opds -l fr --category=Bon_pour_export","message" => "Page not found for: Balzac,_sa_méthode_de_travail"]
In Api.php line 293:
Page not found for: Balzac,_sa_méthode_de_travail
app:opds [-l|--lang LANG] [-c|--category CATEGORY]