flashcache_destroy: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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:

/sbin/flashcache_destroy - from package: flashcache-utils

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