x86_64-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/x86_64-w64-mingw32-as - from package: mingw64-binutils

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

On Debian possible default full paths are:

/usr/bin/x86_64-w64-mingw32-as - from package: binutils-mingw-w64-x86-64

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

Error in other languages:
x86_64-w64-mingw32-as: bevel nie gevind nie
x86_64-w64-mingw32-as: comandă negăsită
x86_64-w64-mingw32-as: comando não encontrado
x86_64-w64-mingw32-as: commande introuvable
x86_64-w64-mingw32-as: command not found
x86_64-w64-mingw32-as: käsku ei ole
x86_64-w64-mingw32-as: không tìm thấy lệnh
x86_64-w64-mingw32-as: komanda nerasta
x86_64-w64-mingw32-as: Kommando nicht gefunden.
x86_64-w64-mingw32-as: kommandot finns inte
x86_64-w64-mingw32-as: komut yok
x86_64-w64-mingw32-as: nie znaleziono polecenia
x86_64-w64-mingw32-as: níor aimsíodh an t-ordú
x86_64-w64-mingw32-as: no se encontró la orden
x86_64-w64-mingw32-as: no s'ha trobat l'ordre
x86_64-w64-mingw32-as: opdracht niet gevonden
x86_64-w64-mingw32-as: parancs nem található
x86_64-w64-mingw32-as: perintah tidak ditemukan
x86_64-w64-mingw32-as: príkaz nenájdený
x86_64-w64-mingw32-as: příkaz nenalezen
x86_64-w64-mingw32-as: команда не найдена
x86_64-w64-mingw32-as: командата не е открита
x86_64-w64-mingw32-as: 命令找不到
x86_64-w64-mingw32-as: コマンドが見つかりません