avahi-set-host-name: 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/avahi-set-host-name - from package: avahi-tools

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

On Debian possible default full paths are:

/usr/bin/avahi-set-host-name - from package: avahi-utils

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

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