airspy_si5351c: 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_si5351c - from package: airspy

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

Error in other languages:
airspy_si5351c: bevel nie gevind nie
airspy_si5351c: comandă negăsită
airspy_si5351c: comando não encontrado
airspy_si5351c: commande introuvable
airspy_si5351c: command not found
airspy_si5351c: käsku ei ole
airspy_si5351c: không tìm thấy lệnh
airspy_si5351c: komanda nerasta
airspy_si5351c: Kommando nicht gefunden.
airspy_si5351c: kommandot finns inte
airspy_si5351c: komut yok
airspy_si5351c: nie znaleziono polecenia
airspy_si5351c: níor aimsíodh an t-ordú
airspy_si5351c: no se encontró la orden
airspy_si5351c: no s'ha trobat l'ordre
airspy_si5351c: opdracht niet gevonden
airspy_si5351c: parancs nem található
airspy_si5351c: perintah tidak ditemukan
airspy_si5351c: príkaz nenájdený
airspy_si5351c: příkaz nenalezen
airspy_si5351c: команда не найдена
airspy_si5351c: командата не е открита
airspy_si5351c: 命令找不到
airspy_si5351c: コマンドが見つかりません