sdmem: 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/sdmem - from package: secure-delete

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

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