reboot: 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:

/sbin/reboot - from package: upstart
/usr/bin/reboot - from package: usermode

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

On Debian possible default full paths are:

/sbin/reboot - from package: systemd-sysv
/sbin/reboot - from package: sysvinit-core
/sbin/reboot - from package: upstart
/usr/lib/klibc/bin/reboot - from package: klibc-utils
/usr/sbin/reboot - from package: molly-guard

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

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