Hello all! I am the maintainer of the YunoHost Flarum package. Sorry to have missed this thread.
flarumer1, if you encounter errors with Flarum on YunoHost, please first check if it is not YunoHost's fault first. 🙂
It is not explicitely stated anywhere, my bad, but composer is actually installed in Flarum's directory. The following commands would have worked:
cd /var/www/flarum
./composer.phar require sijad/flarum-ext-pages
Anyway, as others stated, with the inclusion of Bazaar directly upon the app installation, there should be minimal need for CLI now. 🙂