If you're getting an error "could not import 1 of 1" when dragging mp4 video files into Reaper the solution is very simple.
Just install the 64-bit version of VLC Media Player. Don't just click "Download" - you need to click the triangle to open the drop-down menu to get the 64-bit installer package for VLC.
https://www.videolan.org/vlc/download...