fvwm-menu-xlock: 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/fvwm-menu-xlock - from package: fvwm

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

On Debian possible default full paths are:

/usr/bin/fvwm-menu-xlock - from package: fvwm

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

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