gr_plot_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/gr_plot_fft - from package: gnuradio

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

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