xtightvncviewer: 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/xtightvncviewer - from package: xtightvncviewer

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

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