seahorse-preferences: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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/seahorse-preferences - from package: seahorse-plugins

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