blkdiscard: 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 CentOS possible default full paths are:

/sbin/blkdiscard - from package: util-linux-ng

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/sbin/blkdiscard - from package: util-linux

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

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