问题是您的create路由在常规posts/{id}路由之后列出. Laravel将/posts/之后的每个段都作为ID处理，并忽略特殊" create路由.

The problem is that your create route is listed after the general posts/{id} route. Laravel handles every segment after the /posts/ as an id and ignores the "special" create route.

像这样交换这两行:

Route::get('/posts/create', 'PostController@showForm');
Route::get('/posts/{id}', 'PostController@showById');

此外，我建议您使用资源控制器，这会使您路由使生活变得更轻松！

Furthermore I'd suggest you to use resource controllers which makes your life a bit easier with routing!