plserver: 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 CentOS possible default full paths are:

/usr/bin/plserver - from package: plplot-tk

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/lib/x86_64-linux-gnu/plplot/bin/plserver - from package: plplot-tcl

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

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