rtcwake: 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/sbin/rtcwake - from package: util-linux-ng

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

On Debian possible default full paths are:

/usr/sbin/rtcwake - from package: util-linux

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

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