xpce: 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/xpce - from package: pl

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

On Debian possible default full paths are:

/usr/bin/xpce - from package: swi-prolog-x

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

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