rtl_sdr: 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/rtl_sdr - from package: rtl-sdr

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

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