pkisilent: 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/pkisilent - from package: pki-silent

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

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