Открываем файл со списком репозиториев:
nano /etc/apt/sources.list
Вставить туда следующее содержимое:
deb http://ftp.debian.org/debian bullseye main contrib non-free deb-src http://ftp.debian.org/debian bullseye main contrib non-free deb http://ftp.debian.org/debian bullseye-updates main contrib non-free deb-src http://ftp.debian.org/debian bullseye-updates main contrib non-free deb http://security.debian.org/debian-security bullseye-security main contrib non-free deb-src http://security.debian.org/debian-security bullseye-security main contrib non-free deb http://ftp.debian.org/debian bullseye-backports main contrib non-free