Mirzaagha your command is wrong, clark showed you here clarkwinkelmann the command you need to use.
cd /home/codestac/public_html && /usr/local/bin/php flarum schedule:run > /dev/null
The path /home/codestac/public_html
assumes your flarum file is located in there. Mine isn't setup quite the same way, so just make sure you are not exposing your config.php
or any other sensitive configurations in public_html
.