Try this:
sudo -i
source /usr/local/hestia/conf/hestia.conf
COMPOSER_ALLOW_SUPERUSER=1 "/home/$ROOT_USER/.composer/composer" require symfony/mime --working-dir=/usr/local/hestia/web/fm
Try this:
sudo -i
source /usr/local/hestia/conf/hestia.conf
COMPOSER_ALLOW_SUPERUSER=1 "/home/$ROOT_USER/.composer/composer" require symfony/mime --working-dir=/usr/local/hestia/web/fm