pm_which: 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/pm_which - from package: libmodule-util-perl

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

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