ip6topk: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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/ip6topk - from package: cjdns-python

You may install the required package with command yum install {package}
Error in other languages:
ip6topk: bevel nie gevind nie
ip6topk: comandă negăsită
ip6topk: comando não encontrado
ip6topk: commande introuvable
ip6topk: command not found
ip6topk: käsku ei ole
ip6topk: không tìm thấy lệnh
ip6topk: komanda nerasta
ip6topk: Kommando nicht gefunden.
ip6topk: kommandot finns inte
ip6topk: komut yok
ip6topk: nie znaleziono polecenia
ip6topk: níor aimsíodh an t-ordú
ip6topk: no se encontró la orden
ip6topk: no s'ha trobat l'ordre
ip6topk: opdracht niet gevonden
ip6topk: parancs nem található
ip6topk: perintah tidak ditemukan
ip6topk: príkaz nenájdený
ip6topk: příkaz nenalezen
ip6topk: команда не найдена
ip6topk: командата не е открита
ip6topk: 命令找不到
ip6topk: コマンドが見つかりません