x86_64-w64-mingw32-gfortran-posix: 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 Debian possible default full paths are:

/usr/bin/x86_64-w64-mingw32-gfortran-posix - from package: gfortran-mingw-w64-x86-64

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

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