FFmpeg and VLC on ReactOS

FFmpeg

Currently ReactOS only covers through Windows XP/2003. FFmpeg has required Windows 7 or newer since 2015. Thus without some hacks or compiling FFmpeg on ReactOS yourself, FFmpeg isn’t currently available on ReactOS.

VLC

A current version of VLC is in the ReactOS Application Explorer, try doing what you need with VLC instead of FFmpeg with ReactOS until ReactOS can support Windows 7 applications better.