apt: command not found

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/bin/apt - from package: apt
/usr/lib/debug/usr/lib/jvm/java-7-openjdk-amd64/bin/apt - from package: openjdk-7-dbg
/usr/lib/jvm/java-7-openjdk-amd64/bin/apt - from package: openjdk-7-jdk

You may install the required package with command apt-get install {package}

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