rrdtool: 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/rrdtool - from package: rrdtool

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

On Debian possible default full paths are:

/usr/bin/rrdtool - from package: rrdtool

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

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