Hi !
I am the maintainer of the Flarum package for YunoHost. It seems I only posted the relevant information on GitHub, and forgot about the forums. So yes, when upgrading the package extensions are lost.
Regarding performing the installation commands, my guess is that @Aristid was not using the correct PHP version neither the correct user. Currently the default PHP version shipped with YunoHost is 7.0, hence php
points to php7.0
, thus we get those errors.
cd /var/www/flarum
sudo -u flarum php7.3 composer.phar require fof/upload