plex-2.6.4: 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 Debian possible default full paths are:

/usr/bin/plex-2.6.4 - from package: fp-utils-2.6.4

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

Error in other languages:
plex-2.6.4: bevel nie gevind nie
plex-2.6.4: comandă negăsită
plex-2.6.4: comando não encontrado
plex-2.6.4: commande introuvable
plex-2.6.4: command not found
plex-2.6.4: käsku ei ole
plex-2.6.4: không tìm thấy lệnh
plex-2.6.4: komanda nerasta
plex-2.6.4: Kommando nicht gefunden.
plex-2.6.4: kommandot finns inte
plex-2.6.4: komut yok
plex-2.6.4: nie znaleziono polecenia
plex-2.6.4: níor aimsíodh an t-ordú
plex-2.6.4: no se encontró la orden
plex-2.6.4: no s'ha trobat l'ordre
plex-2.6.4: opdracht niet gevonden
plex-2.6.4: parancs nem található
plex-2.6.4: perintah tidak ditemukan
plex-2.6.4: príkaz nenájdený
plex-2.6.4: příkaz nenalezen
plex-2.6.4: команда не найдена
plex-2.6.4: командата не е открита
plex-2.6.4: 命令找不到
plex-2.6.4: コマンドが見つかりません