xmonad: 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/xmonad - from package: xmonad-core

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

On Debian possible default full paths are:

/usr/bin/xmonad - from package: xmonad

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

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