Hi @2jiwon,
This shouldn't be a very big deal.
First, if you do sudo rm /etc/apache2/sites-available/certbot.log, you will get rid of the syntax error. This is a file created by Certbot but does not belong in that directory at all. If you want to move it instead, you could do
sudo mv -i /etc/apache2/sites-available/certbot.log ~
Then, you might want to know about the reason behind the error about the authenticator's challenge support.
(Using the standalone authenticator plugin will bypass this, but it might be more convenient to use the Apache authenticator plugin, the way you were trying to do, particularly since it doesn't require the hooks to start and stop the web server.)