list_instances: 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/list_instances - from package: python2-boto
/usr/bin/list_instances - from package: python-boto

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

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