union: 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/union - from package: emboss

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

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