sleepctl: 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 Debian possible default full paths are:

/usr/bin/sleepctl - from package: sleepd

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

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