dnssec-system-tray: 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/dnssec-system-tray - from package: dnssec-system-tray

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

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