amavisd-agent: 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/amavisd-agent - from package: amavisd-new

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

On Debian possible default full paths are:

/usr/sbin/amavisd-agent - from package: amavisd-new

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

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