after installing ,page appears with this error message, any help?

Just ran into another problem, seems like theres here:

Fatal error: Call to undefined function Composer\Autoload\includeFile() in /flarum/vendor/composer/ClassLoader.php on line 301

Did you run composer as mentioned in the Installation manual? And did this result in warnings or error messages?

Let's keep this to your existing thread. Locking this one.