It seems like there has been an attempt to enable mpm_event with php-fpm.
Do the following:
a2dismod mpm_event
# If it prints an error that other modules depend on mpm_event, disable them the same way first
a2enmod mpm_prefork
a2enmod php7.3
# If it still shows php7.3 module missing, try to reinstall the module
apt install --reinstall libapache2-mod-php7.3
a2enmod php7.3
systemctl restart apache2