gr_psd_plot_s: 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_s - from package: gnuradio

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

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