xfce4-panel: 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/xfce4-panel - from package: xfce4-panel

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

On Debian possible default full paths are:

/usr/bin/xfce4-panel - from package: xfce4-panel

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

Error in other languages:
xfce4-panel: bevel nie gevind nie
xfce4-panel: comandă negăsită
xfce4-panel: comando não encontrado
xfce4-panel: commande introuvable
xfce4-panel: command not found
xfce4-panel: käsku ei ole
xfce4-panel: không tìm thấy lệnh
xfce4-panel: komanda nerasta
xfce4-panel: Kommando nicht gefunden.
xfce4-panel: kommandot finns inte
xfce4-panel: komut yok
xfce4-panel: nie znaleziono polecenia
xfce4-panel: níor aimsíodh an t-ordú
xfce4-panel: no se encontró la orden
xfce4-panel: no s'ha trobat l'ordre
xfce4-panel: opdracht niet gevonden
xfce4-panel: parancs nem található
xfce4-panel: perintah tidak ditemukan
xfce4-panel: príkaz nenájdený
xfce4-panel: příkaz nenalezen
xfce4-panel: команда не найдена
xfce4-panel: командата не е открита
xfce4-panel: 命令找不到
xfce4-panel: コマンドが見つかりません