You can also try to reinstall Apache2 first: apt install --reinstall apache2-data apache2 libapache2-mod-php7.3
I have no idea how this can happen, but contained files/dirs are still missing in your case. The following dir must exist:
/etc/apache2/mods-available/ (part of Apache2 package)
and ist must contain a php7.3.conf file (part of libapache2-mod-php7.3)
Then a2enmod php7.3 must succeed, to have Apache2 handle PHP files correctly.
If this still is not the case, then there must be something wrong deeper in your system, e.g. file corruption, broken apt or dpkg or such. Probably dmesg then shows some related file system errors or such.