rrdupdate: 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/rrdupdate - from package: rrdtool

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

On Debian possible default full paths are:

/usr/bin/rrdupdate - from package: rrdtool

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

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