tightvncpasswd: 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/tightvncpasswd - from package: tightvncserver

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

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