zynaddsubfx: command not found

Have You tried accessing the binary with the full path?

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/zynaddsubfx - from package: zynaddsubfx

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