xpce-client: 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/lib64/pl-5.7.11/xpce-6.6.65/bin/x86_64-linux/xpce-client - from package: pl
/usr/lib/pl-5.7.11/xpce-6.6.65/bin/i686-linux/xpce-client - from package: pl

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

On Debian possible default full paths are:

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

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

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