gr_psd_plot_b: 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_psd_plot_b - from package: gnuradio

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

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