airspy_r820t: 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/airspy_r820t - from package: airspy

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

Error in other languages:
airspy_r820t: bevel nie gevind nie
airspy_r820t: comandă negăsită
airspy_r820t: comando não encontrado
airspy_r820t: commande introuvable
airspy_r820t: command not found
airspy_r820t: käsku ei ole
airspy_r820t: không tìm thấy lệnh
airspy_r820t: komanda nerasta
airspy_r820t: Kommando nicht gefunden.
airspy_r820t: kommandot finns inte
airspy_r820t: komut yok
airspy_r820t: nie znaleziono polecenia
airspy_r820t: níor aimsíodh an t-ordú
airspy_r820t: no se encontró la orden
airspy_r820t: no s'ha trobat l'ordre
airspy_r820t: opdracht niet gevonden
airspy_r820t: parancs nem található
airspy_r820t: perintah tidak ditemukan
airspy_r820t: príkaz nenájdený
airspy_r820t: příkaz nenalezen
airspy_r820t: команда не найдена
airspy_r820t: командата не е открита
airspy_r820t: 命令找不到
airspy_r820t: コマンドが見つかりません