pkill: 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/pkill - from package: procps

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

On Debian possible default full paths are:

/usr/bin/pkill - from package: procps

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

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