apt-cacher-ng: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/sbin/apt-cacher-ng - from package: apt-cacher-ng

You may install the required package with command apt-get install {package}
Error in other languages:
apt-cacher-ng: bevel nie gevind nie
apt-cacher-ng: comandă negăsită
apt-cacher-ng: comando não encontrado
apt-cacher-ng: commande introuvable
apt-cacher-ng: command not found
apt-cacher-ng: käsku ei ole
apt-cacher-ng: không tìm thấy lệnh
apt-cacher-ng: komanda nerasta
apt-cacher-ng: Kommando nicht gefunden.
apt-cacher-ng: kommandot finns inte
apt-cacher-ng: komut yok
apt-cacher-ng: nie znaleziono polecenia
apt-cacher-ng: níor aimsíodh an t-ordú
apt-cacher-ng: no se encontró la orden
apt-cacher-ng: no s'ha trobat l'ordre
apt-cacher-ng: opdracht niet gevonden
apt-cacher-ng: parancs nem található
apt-cacher-ng: perintah tidak ditemukan
apt-cacher-ng: príkaz nenájdený
apt-cacher-ng: příkaz nenalezen
apt-cacher-ng: команда не найдена
apt-cacher-ng: командата не е открита
apt-cacher-ng: 命令找不到
apt-cacher-ng: コマンドが見つかりません