Fresh nextcloud install failed

Troubleshooting
11

What the hack… This module is fixed part of the package and must be available: https://packages.debian.org/buster/amd64/libapache2-mod-php7.3/filelist
I checked the postinst script:

#!/bin/sh

set -e

if [ -e /usr/share/apache2/apache2-maintscript-helper ]; then
    . /usr/share/apache2/apache2-maintscript-helper

    php_enable() {
	local a2query_ret=0
	a2query -m "php7.3" > /dev/null 2>&1 || a2query_ret=$?
	if [ "$a2query_ret" -eq 0 ] ; then
	    apache2_msg "info" "$DPKG_MAINTSCRIPT_PACKAGE: not switching MPM - already enabled"
	    return 1
	elif [ "$a2query_ret" -eq 32	 ] ; then
	    apache2_msg "info" "$DPKG_MAINTSCRIPT_PACKAGE: not switching MPM - disabled by maintainer"
	    return 1
	fi

	PHP_MODULE=$(a2query -m | sed -n 's/^\(php[\.0-9]*\) (enabled.*)/\1/p')
        if [ -n "$PHP_MODULE" -a "$PHP_MODULE" != "php7.3" ]; then
	    apache2_msg "err" "$DPKG_MAINTSCRIPT_PACKAGE: $PHP_MODULE module already enabled, not enabling PHP 7.3"
	    return 1
	fi

	mpm=$(a2query -M)
	case "$(a2query -M)" in
	    prefork|itk) return 0;;
	    *) if apache2_switch_mpm prefork; then return 0; fi;;
	esac
        apache2_msg "err" "$DPKG_MAINTSCRIPT_PACKAGE: Could not switch to prefork MPM, not enabling PHP 7.3"
	return 1
    }

else
    echo "Warning: Could not load Apache 2.4 maintainer script helper."

    php_enable() {
        return 1;
    }

fi

# we've registered a trigger to handle extension updates.
if [ "$1" = "triggered" ] && [ "$2" = "/etc/php/7.3/apache2/conf.d" ]; then
    if [ -e /usr/share/apache2/apache2-maintscript-helper ]; then
	. /usr/share/apache2/apache2-maintscript-helper
	apache2_reload restart
    fi
    exit 0
elif [ "$1" != "configure" ]; then
    exit 0
fi

CONFDIR=/etc/php/7.3/apache2/conf.d

if [ "$1" = "configure" ]; then
    phpini="/etc/php/7.3/apache2/php.ini"

    if [ "apache2" = "cli" ]; then
	ucf /usr/lib/php/7.3/php.ini-production.apache2 $phpini
    else
	ucf /usr/lib/php/7.3/php.ini-production $phpini
    fi
    ucfr libapache2-mod-php7.3 $phpini

    if [ -e /usr/lib/php/php-maintscript-helper ]; then
	. /usr/lib/php/php-maintscript-helper
	
	mods=$(phpquery -M -v 7.3)
	for mod in $mods; do
	    php_invoke enmod 7.3 apache2 $mod
	done
    fi

    if [ -n "$2" ] && dpkg --compare-versions "$2" lt "7.0.3-2~"; then
	rm -f /etc/php/7.3/apache2/conf.d/20-opcache.ini;
    fi	    
fi

# Automatically added by dh_apache2/UNDECLARED
if php_enable; then
	if [ -e /usr/share/apache2/apache2-maintscript-helper ] ; then
		. /usr/share/apache2/apache2-maintscript-helper
		for conf in php7.3  ; do
			apache2_invoke enmod $conf  || exit 1
		done
	fi
fi
# End automatically added section


exit 0

Only chance it fails can be just for the same reason, php7.3 module not being available.

Lets debug from start. Could you paste:

dpkg -l | grep -E '(php|apache)'
ls -l /etc/apache2/mods-available/php*