lvextend: 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:

/sbin/lvextend - from package: lvm2

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

On Debian possible default full paths are:

/sbin/lvextend - from package: lvm2

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

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