gr_plot_fft_c: 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_c - from package: gnuradio

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

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