expire_backups: 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/bin/expire_backups - from package: s3ql

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

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