rings3d: 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 Debian possible default full paths are:

/usr/bin/rings3d - from package: raster3d

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

Error in other languages:
rings3d: bevel nie gevind nie
rings3d: comandă negăsită
rings3d: comando não encontrado
rings3d: commande introuvable
rings3d: command not found
rings3d: käsku ei ole
rings3d: không tìm thấy lệnh
rings3d: komanda nerasta
rings3d: Kommando nicht gefunden.
rings3d: kommandot finns inte
rings3d: komut yok
rings3d: nie znaleziono polecenia
rings3d: níor aimsíodh an t-ordú
rings3d: no se encontró la orden
rings3d: no s'ha trobat l'ordre
rings3d: opdracht niet gevonden
rings3d: parancs nem található
rings3d: perintah tidak ditemukan
rings3d: príkaz nenájdený
rings3d: příkaz nenalezen
rings3d: команда не найдена
rings3d: командата не е открита
rings3d: 命令找不到
rings3d: コマンドが見つかりません