This may be a bug.

Steps to replicate it:

Disable IPv6 in the kernel via GRUB

GRUB_CMDLINE_LINUX="ipv6.disable=1"

Install WordOps and the web stack

wget -qO wo wops.cc && sudo bash wo
wo stack install --web

Note: WordOps is installed successfully however the web stack fails to install

The log from the failed web stack installation

root@ubuntu:_# tail /var/log/wo/wordops.log
Processing triggers for libc-bin (2.27-3ubuntu1) ...
Processing triggers for systemd (237-3ubuntu10.29) ...
Processing triggers for man-db (2.8.3-2ubuntu0.1) ...
Processing triggers for ufw (0.36-0ubuntu0.18.04.1) ...
Processing triggers for php7.2-fpm (7.2.22-1+ubuntu18.04.1+deb.sury.org+1) ...
Errors were encountered while processing:
nginx-custom
E: Sub-process /usr/bin/dpkg returned an error code (1)
2019-10-06 16:30:06,031 (INFO) wo : Oops Something went wrong!!
2019-10-06 16:30:06,032 (ERROR) wo : Check the WordOps log for more details tail /var/log/wo/wordops.log and please try again...

The culprit is nginx-custom and testing the nginx config reveals

root@ubuntu:_# nginx -c /etc/nginx/nginx.conf -t
nginx: the configuration file /etc/nginx/nginx.conf syntax is ok
nginx: [emerg] socket() [::]:80 failed (97: Address family not supported by protocol)
nginx: configuration file /etc/nginx/nginx.conf test failed

After I reverted the changes in GRUB, I was able install the web stack without any issues

root@ubuntu:_# wo stack install --web
Adding repository for MySQL, please wait...
Adding repository for NGINX, please wait...
Adding repository for PHP, please wait...
Updating apt-cache [OK]
Installing APT packages [OK]
Applying Nginx configuration templates
Testing Nginx configuration [OK]
Restarting Nginx [OK]
Configuring php7.2-fpm
Restarting php7.2-fpm [OK]
Tuning MariaDB configuration
Stop : mysql [OK]
Start : mysql [OK]
Configuring APT packages [OK]
Downloading WP-CLI [Done]
Downloading MySQLTuner [Done]
Configuring packages [OK]