backup-whatsthis: 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/backup-whatsthis - from package: chiark-backup

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

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