gnuplot5-x11: 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-x11 - from package: gnuplot5-x11

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

Error in other languages:
gnuplot5-x11: bevel nie gevind nie
gnuplot5-x11: comandă negăsită
gnuplot5-x11: comando não encontrado
gnuplot5-x11: commande introuvable
gnuplot5-x11: command not found
gnuplot5-x11: käsku ei ole
gnuplot5-x11: không tìm thấy lệnh
gnuplot5-x11: komanda nerasta
gnuplot5-x11: Kommando nicht gefunden.
gnuplot5-x11: kommandot finns inte
gnuplot5-x11: komut yok
gnuplot5-x11: nie znaleziono polecenia
gnuplot5-x11: níor aimsíodh an t-ordú
gnuplot5-x11: no se encontró la orden
gnuplot5-x11: no s'ha trobat l'ordre
gnuplot5-x11: opdracht niet gevonden
gnuplot5-x11: parancs nem található
gnuplot5-x11: perintah tidak ditemukan
gnuplot5-x11: príkaz nenájdený
gnuplot5-x11: příkaz nenalezen
gnuplot5-x11: команда не найдена
gnuplot5-x11: командата не е открита
gnuplot5-x11: 命令找不到
gnuplot5-x11: コマンドが見つかりません