gr_spectrogram_plot_i: 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_spectrogram_plot_i - from package: gnuradio

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

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