gr_constellation_plot: 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_constellation_plot - from package: gnuradio

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

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