lvreduce: 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/lvreduce - from package: lvm2

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

On Debian possible default full paths are:

/sbin/lvreduce - from package: lvm2

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

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