sleep: 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:

/bin/sleep - from package: coreutils

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

On Debian possible default full paths are:

/bin/sleep - from package: coreutils
/usr/lib/plan9/bin/sleep - from package: 9base
/usr/lib/klibc/bin/sleep - from package: klibc-utils

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

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