hackrf_transfer: 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/hackrf_transfer - from package: hackrf

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

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