parkill: 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/parkill - from package: mpich2
/usr/lib64/mpich/bin/parkill - from package: mpich
/usr/lib/mpich/bin/parkill - from package: mpich
/usr/lib64/mvapich2-psm/bin/parkill - from package: mvapich2-psm
/usr/lib64/mvapich2/bin/parkill - from package: mvapich2
/usr/lib/mvapich2/bin/parkill - from package: mvapich2

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

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