plasmoidviewer: 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/plasmoidviewer - from package: kdebase-workspace

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

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