rltraceroute6: 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/sbin/rltraceroute6 - from package: ndisc6

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

On Debian possible default full paths are:

/usr/bin/rltraceroute6 - from package: ndisc6

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

Error in other languages:
rltraceroute6: bevel nie gevind nie
rltraceroute6: comandă negăsită
rltraceroute6: comando não encontrado
rltraceroute6: commande introuvable
rltraceroute6: command not found
rltraceroute6: käsku ei ole
rltraceroute6: không tìm thấy lệnh
rltraceroute6: komanda nerasta
rltraceroute6: Kommando nicht gefunden.
rltraceroute6: kommandot finns inte
rltraceroute6: komut yok
rltraceroute6: nie znaleziono polecenia
rltraceroute6: níor aimsíodh an t-ordú
rltraceroute6: no se encontró la orden
rltraceroute6: no s'ha trobat l'ordre
rltraceroute6: opdracht niet gevonden
rltraceroute6: parancs nem található
rltraceroute6: perintah tidak ditemukan
rltraceroute6: príkaz nenájdený
rltraceroute6: příkaz nenalezen
rltraceroute6: команда не найдена
rltraceroute6: командата не е открита
rltraceroute6: 命令找不到
rltraceroute6: コマンドが見つかりません