amavisd-signer: 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/sbin/amavisd-signer - from package: amavisd-new

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

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