rrdcgi: 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/rrdcgi - from package: rrdtool

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

On Debian possible default full paths are:

/usr/bin/rrdcgi - from package: rrdtool

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

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