NFGamerMC Looks like Flarum didn't install properly.
Something went wrong during the installation process. Double check that your server fulfills all the system requirements and then try running php flarum install to finish.
If the error still persists, try deleting the vendor folder and run composer install. It's possible not all dependencies and Flarum files finished downloading.
If none of this works, I'd recommend deleting the folder you installed flarum to and running the install command again.
