xscreensaver-command: 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/xscreensaver-command - from package: xscreensaver-base

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

On Debian possible default full paths are:

/usr/bin/xscreensaver-command - from package: xscreensaver

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

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