sigma: 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/sigma - from package: sigma-align

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

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