plasma-desktop: 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/plasma-desktop - from package: kdebase-workspace

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

On Debian possible default full paths are:

/usr/bin/plasma-desktop - from package: plasma-desktop

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

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