spamassassin: 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/spamassassin - from package: spamassassin

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

On Debian possible default full paths are:

/usr/bin/spamassassin - from package: spamassassin

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

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