blockmode: 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:

/usr/bin/blockmode - from package: GMT

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

On Debian possible default full paths are:

/usr/lib/gmt/bin/blockmode - from package: GMT

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

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