ack-grep: 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/ack-grep - from package: ack-grep

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

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