collectd-nagios: 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/collectd-nagios - from package: collectd

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

On Debian possible default full paths are:

/usr/bin/collectd-nagios - from package: collectd-utils

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

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