xmonad-start: 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-start - from package: xmonad-core

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

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