删除话题和删除回复的路由命名不一致
// 删除回复
$api->delete('topics/{topic}/replies/{reply}', 'RepliesController@destroy')
->name('api.topics.replies.destroy');
// 删除话题
$api->delete('topics/{topic}', 'TopicsController@destroy')
->name('api.topics.delete');
推荐文章: