sa-update: 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 CentOS possible default full paths are:

/usr/bin/sa-update - from package: spamassassin

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/sa-update - from package: spamassassin

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

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