ltsp-open: 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/ltsp-open - from package: ltsp-client-core

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

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