rrd2whisper: 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/rrd2whisper - from package: python-whisper

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

On Debian possible default full paths are:

/usr/bin/rrd2whisper - from package: python-whisper

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

Error in other languages:
rrd2whisper: bevel nie gevind nie
rrd2whisper: comandă negăsită
rrd2whisper: comando não encontrado
rrd2whisper: commande introuvable
rrd2whisper: command not found
rrd2whisper: käsku ei ole
rrd2whisper: không tìm thấy lệnh
rrd2whisper: komanda nerasta
rrd2whisper: Kommando nicht gefunden.
rrd2whisper: kommandot finns inte
rrd2whisper: komut yok
rrd2whisper: nie znaleziono polecenia
rrd2whisper: níor aimsíodh an t-ordú
rrd2whisper: no se encontró la orden
rrd2whisper: no s'ha trobat l'ordre
rrd2whisper: opdracht niet gevonden
rrd2whisper: parancs nem található
rrd2whisper: perintah tidak ditemukan
rrd2whisper: príkaz nenájdený
rrd2whisper: příkaz nenalezen
rrd2whisper: команда не найдена
rrd2whisper: командата не е открита
rrd2whisper: 命令找不到
rrd2whisper: コマンドが見つかりません