sa-learn: 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-learn - from package: spamassassin

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

On Debian possible default full paths are:

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

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

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