zsync: 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/zsync - from package: zsync

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

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