>>282
echo "deb http://download.opensuse.org/repositories/home:/Sokoloff/Debian_10 ./" > /etc/apt/sources.list.d/flacon.list

↑これがpermission deniedってこと？

それだったら
echo "deb http://download.opensuse.org/repositories/home:/Sokoloff/Debian_10 ./" | tee -a /etc/apt/sources.list.d/flacon.list

的外れだったらごめん。