ppl_pl: 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/ppl_pl - from package: ppl-swiprolog

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

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