Administration - Installation - Core: Re: ILIAS installation Error (autoload.php)

the instructions on https://bbs-ilias.de/ilias/ilias.php?ref_id=62&bmn=2020-09&blpg=13&cmd=previewFullscreen&cmdClass=ilblogpostinggui&cmdNode=dx:z0:yz&baseClass=ilrepositorygui
are much more complete than the official ones
the error you get is because you haven't built static artifacts from composer needed for ilias 6 (this is not explained even on the official documentation)
follow these steps
Download and install composer
cd /home/youruser/somefolder
download this https://getcomposer.org/download/1.9.3/composer.phar
sudo mv composer.phar /usr/local/bin/composer
sudo chmod +x /usr/local/bin/composer

Change temporarly ownership user/group so composer can write as non-root following instrucions below! 
sudo chown youruser:youruser /var/www/html/ilias
sudo find /var/www/html -type d -exec chmod 0755 {} \;
sudo find /var/www/html -type f -exec chmod 0644 {} \;
sudo find /var/www/html/ilias -type d -exec chmod 0755 {} \;
sudo find /var/www/html/ilias -type f -exec chmod 0644 {} \;

Setup static artifacts via composer:
cd /var/www/html/ilias
composer update --no-dev

sudo chown www-data:www-data /var/www/html/ilias
sudo find /var/www/html/ilias -type d -exec chmod 0755 {} \;
sudo find /var/www/html/ilias -type f -exec chmod 0644 {} \;

sudo systemctl restart apache2.service

Retry
If DB doesn't connect during installation do yourself a favor and switch to mysql 5.6 or 5.7 or mariadb

Cheers