i686-w64-mingw32-as: 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/i686-w64-mingw32-as - from package: mingw32-binutils

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

On Debian possible default full paths are:

/usr/bin/i686-w64-mingw32-as - from package: binutils-mingw-w64-i686

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

Error in other languages:
i686-w64-mingw32-as: bevel nie gevind nie
i686-w64-mingw32-as: comandă negăsită
i686-w64-mingw32-as: comando não encontrado
i686-w64-mingw32-as: commande introuvable
i686-w64-mingw32-as: command not found
i686-w64-mingw32-as: käsku ei ole
i686-w64-mingw32-as: không tìm thấy lệnh
i686-w64-mingw32-as: komanda nerasta
i686-w64-mingw32-as: Kommando nicht gefunden.
i686-w64-mingw32-as: kommandot finns inte
i686-w64-mingw32-as: komut yok
i686-w64-mingw32-as: nie znaleziono polecenia
i686-w64-mingw32-as: níor aimsíodh an t-ordú
i686-w64-mingw32-as: no se encontró la orden
i686-w64-mingw32-as: no s'ha trobat l'ordre
i686-w64-mingw32-as: opdracht niet gevonden
i686-w64-mingw32-as: parancs nem található
i686-w64-mingw32-as: perintah tidak ditemukan
i686-w64-mingw32-as: príkaz nenájdený
i686-w64-mingw32-as: příkaz nenalezen
i686-w64-mingw32-as: команда не найдена
i686-w64-mingw32-as: командата не е открита
i686-w64-mingw32-as: 命令找不到
i686-w64-mingw32-as: コマンドが見つかりません