lxc-attach: 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/lxc-attach - from package: lxc

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

On Debian possible default full paths are:

/usr/bin/lxc-attach - from package: lxc

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

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