task_manager: 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/task_manager - from package: sympa

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

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