synthcal: 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/synthcal - from package: argyll

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

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