gnuplot5-nox: 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/gnuplot5-nox - from package: gnuplot5-nox

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

Error in other languages:
gnuplot5-nox: bevel nie gevind nie
gnuplot5-nox: comandă negăsită
gnuplot5-nox: comando não encontrado
gnuplot5-nox: commande introuvable
gnuplot5-nox: command not found
gnuplot5-nox: käsku ei ole
gnuplot5-nox: không tìm thấy lệnh
gnuplot5-nox: komanda nerasta
gnuplot5-nox: Kommando nicht gefunden.
gnuplot5-nox: kommandot finns inte
gnuplot5-nox: komut yok
gnuplot5-nox: nie znaleziono polecenia
gnuplot5-nox: níor aimsíodh an t-ordú
gnuplot5-nox: no se encontró la orden
gnuplot5-nox: no s'ha trobat l'ordre
gnuplot5-nox: opdracht niet gevonden
gnuplot5-nox: parancs nem található
gnuplot5-nox: perintah tidak ditemukan
gnuplot5-nox: príkaz nenájdený
gnuplot5-nox: příkaz nenalezen
gnuplot5-nox: команда не найдена
gnuplot5-nox: командата не е открита
gnuplot5-nox: 命令找不到
gnuplot5-nox: コマンドが見つかりません