munin-cron: 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/munin-cron - from package: munin

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

On Debian possible default full paths are:

/usr/bin/munin-cron - from package: munin

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

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