WPrefs: 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/WPrefs - from package: WindowMaker

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

On Debian possible default full paths are:

/usr/bin/WPrefs - from package: wmaker

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

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