threaded_memtest: 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/lib/plainbox-providers-1/checkbox/bin/threaded_memtest - from package: plainbox-provider-checkbox

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

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