Cannot complete Worpdress installation through web interface

I am hosting a website on my own private server (a raspeberry pi3 running raspbian, based on ubuntu 16.04) on which I installed LAMP (following this step-by-step guide) and wordpress (following this one).

I installed wordpress in a “secondary” site, meaning that it is in a folder that is contained within my “main site”'s folder. The main site location is /var/www/my-main-site/, which contains the folder /var/www/my-main-site/my-secondary-site, where all wordpress files are (below I copied the folder structures and their permissions).

The secondary site can be reached through an alias, which I configured in the /etc/apache2/sites-enabled/my-main-site.conf file as follows

<VirtualHost *:port>

    ServerAdmin my-email@mail.com
    ServerName my-public-ip:port
    Alias "/my-secondary-site" "/var/www/my-main-site/my-secondary-site"
    DocumentRoot /var/www/my-main-site/public_html

    ErrorLog ${APACHE_LOG_DIR}/error.log
    CustomLog ${APACHE_LOG_DIR}/access.log combined

While testing it with two sample html files it all works: I can access my main site through my-public-ip:port and the secondary site through my-public-ip:port/my-secondary-site. After this test, I installed wordpress.

The problem is that when I try to access the secondary site to complete the wordpress configuration online I get this error

Forbidden

You don't have permission to access /my-secondary-site on this server.
Server unable to read htaccess file, denying access to be safe

Following step 3 from the guide I enabled .htaccess overrides by adding these lines at the end of /etc/apache2/apache2.conf

<Directory /var/www/my-main-site/>
    AllowOverride All
</Directory>

Following step5, I changed the permissions for my folders as follows. (note that my-user is the user I use to connect through ssh, which has root privileges)

drwxr-xr-x 4 my-user www-data 4096 May  1 11:48 my-main-site
/* which contains these two folders*/
---> drwxr-xr-x 2 my-user www-data 4096 May  1 11:01 public_html
---> drwxr-sr-x 5 my-user www-data 4096 May  1 14:22 my-secondary-site

/* files and folders in /var/www/my-main-site/my-secondary-site
      ---> -rw-r--r--  1 my-user www-data   147 May  1 11:50 index.html
      ---> -rw-r--r--  1 my-user www-data   418 Sep 25  2013 index.php
      ---> -rw-r--r--  1 my-user www-data 19935 Jan  6 20:32 license.txt
      ---> -rw-r--r--  1 my-user www-data  7415 Mar 18 17:13 readme.html
      ---> -rw-r--r--  1 my-user www-data  5438 Mar 18 16:07 wp-activate.php
      ---> drwxr-sr-x  9 my-user www-data  4096 Apr  3 22:19 wp-admin
      ---> -rw-r--r--  1 my-user www-data   364 Dec 19  2015 wp-blog-header.php
      ---> -rw-r--r--  1 my-user www-data  1627 Aug 29  2016 wp-comments-post.php
      ---> -rw-r--r--  1 my-user www-data  2853 Dec 16  2015 wp-config-sample.php
      ---> -rw-r--r--  1 my-user www-data  3261 May  1 12:59 wp-config.php
      ---> drwxrwsr-x  5 my-user www-data  4096 May  1 12:35 wp-content
      ---> -rw-r--r--  1 my-user www-data  3669 Aug 20  2017 wp-cron.php
      ---> drwxr-sr-x 18 my-user www-data 12288 Apr  3 22:19 wp-includes
      ---> -rw-r--r--  1 my-user www-data  2422 Nov 21  2016 wp-links-opml.php
      ---> -rw-r--r--  1 my-user www-data  3306 Aug 22  2017 wp-load.php
      ---> -rw-r--r--  1 my-user www-data 36593 Apr  3 17:22 wp-login.php
      ---> -rw-r--r--  1 my-user www-data  8048 Jan 11  2017 wp-mail.php
      ---> -rw-r--r--  1 my-user www-data 16246 Oct  4  2017 wp-settings.php
      ---> -rw-r--r--  1 my-user www-data 30071 Oct 18  2017 wp-signup.php
      ---> -rw-r--r--  1 my-user www-data  4620 Oct 24  2017 wp-trackback.php
      ---> -rw-r--r--  1 my-user www-data  3065 Aug 31  2016 xmlrpc.php

I tried to give ownership of /var/www/my-primary-site to the www-data user through sudo chown -R www-data /var/www/my-primary-site/, but the problem persisted.

I allso tried changing the /etc/apache2/apache2.conf file to allow override to the whole /var/www/ folder as follows, but nothing changed.

/* From... */
<Directory /var/www/>
    Options Indexes FollowSymLinks
    AllowOverride None
    Require all granted
</Directory>

/* .... to */
<Directory /var/www/>
    Options Indexes FollowSymLinks
    AllowOverride All
    Require all granted
</Directory>

Any ideas?