ipv6loganon: 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/ipv6loganon - from package: ipv6calc

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

On Debian possible default full paths are:

/usr/bin/ipv6loganon - from package: ipv6calc

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

Error in other languages:
ipv6loganon: bevel nie gevind nie
ipv6loganon: comandă negăsită
ipv6loganon: comando não encontrado
ipv6loganon: commande introuvable
ipv6loganon: command not found
ipv6loganon: käsku ei ole
ipv6loganon: không tìm thấy lệnh
ipv6loganon: komanda nerasta
ipv6loganon: Kommando nicht gefunden.
ipv6loganon: kommandot finns inte
ipv6loganon: komut yok
ipv6loganon: nie znaleziono polecenia
ipv6loganon: níor aimsíodh an t-ordú
ipv6loganon: no se encontró la orden
ipv6loganon: no s'ha trobat l'ordre
ipv6loganon: opdracht niet gevonden
ipv6loganon: parancs nem található
ipv6loganon: perintah tidak ditemukan
ipv6loganon: príkaz nenájdený
ipv6loganon: příkaz nenalezen
ipv6loganon: команда не найдена
ipv6loganon: командата не е открита
ipv6loganon: 命令找不到
ipv6loganon: コマンドが見つかりません