convert2syrthes4: 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/convert2syrthes4 - from package: syrthes-tools
/usr/lib/syrthes/bin/convert2syrthes4 - from package: syrthes-tools

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

Error in other languages:
convert2syrthes4: bevel nie gevind nie
convert2syrthes4: comandă negăsită
convert2syrthes4: comando não encontrado
convert2syrthes4: commande introuvable
convert2syrthes4: command not found
convert2syrthes4: käsku ei ole
convert2syrthes4: không tìm thấy lệnh
convert2syrthes4: komanda nerasta
convert2syrthes4: Kommando nicht gefunden.
convert2syrthes4: kommandot finns inte
convert2syrthes4: komut yok
convert2syrthes4: nie znaleziono polecenia
convert2syrthes4: níor aimsíodh an t-ordú
convert2syrthes4: no se encontró la orden
convert2syrthes4: no s'ha trobat l'ordre
convert2syrthes4: opdracht niet gevonden
convert2syrthes4: parancs nem található
convert2syrthes4: perintah tidak ditemukan
convert2syrthes4: príkaz nenájdený
convert2syrthes4: příkaz nenalezen
convert2syrthes4: команда не найдена
convert2syrthes4: командата не е открита
convert2syrthes4: 命令找不到
convert2syrthes4: コマンドが見つかりません