vtk6: 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/vtk6 - from package: vtk6

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

Error in other languages:
vtk6: bevel nie gevind nie
vtk6: comandă negăsită
vtk6: comando não encontrado
vtk6: commande introuvable
vtk6: command not found
vtk6: käsku ei ole
vtk6: không tìm thấy lệnh
vtk6: komanda nerasta
vtk6: Kommando nicht gefunden.
vtk6: kommandot finns inte
vtk6: komut yok
vtk6: nie znaleziono polecenia
vtk6: níor aimsíodh an t-ordú
vtk6: no se encontró la orden
vtk6: no s'ha trobat l'ordre
vtk6: opdracht niet gevonden
vtk6: parancs nem található
vtk6: perintah tidak ditemukan
vtk6: príkaz nenájdený
vtk6: příkaz nenalezen
vtk6: команда не найдена
vtk6: командата не е открита
vtk6: 命令找不到
vtk6: コマンドが見つかりません