apack: 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 CentOS possible default full paths are:

/usr/bin/apack - from package: atool

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/apack - from package: atool

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

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