rivet-mkhtml: 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/rivet-mkhtml - from package: rivet

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

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