ifnames: 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/ifnames - from package: autoconf

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

On Debian possible default full paths are:

/usr/bin/ifnames - from package: autoconf

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

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