uhd_fft: 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/uhd_fft - from package: gnuradio

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

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