ipv6calc: 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/ipv6calc - from package: ipv6calc

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

On Debian possible default full paths are:

/usr/bin/ipv6calc - from package: ipv6calc

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

Error in other languages:
ipv6calc: bevel nie gevind nie
ipv6calc: comandă negăsită
ipv6calc: comando não encontrado
ipv6calc: commande introuvable
ipv6calc: command not found
ipv6calc: käsku ei ole
ipv6calc: không tìm thấy lệnh
ipv6calc: komanda nerasta
ipv6calc: Kommando nicht gefunden.
ipv6calc: kommandot finns inte
ipv6calc: komut yok
ipv6calc: nie znaleziono polecenia
ipv6calc: níor aimsíodh an t-ordú
ipv6calc: no se encontró la orden
ipv6calc: no s'ha trobat l'ordre
ipv6calc: opdracht niet gevonden
ipv6calc: parancs nem található
ipv6calc: perintah tidak ditemukan
ipv6calc: príkaz nenájdený
ipv6calc: příkaz nenalezen
ipv6calc: команда не найдена
ipv6calc: командата не е открита
ipv6calc: 命令找不到
ipv6calc: コマンドが見つかりません