[libretro ERROR] retro_load_game: couldn't read WAD header from '/home/pi/RetroPie/roms/ports/doom/doom2/DOOM2.WAD'
[INFO] [Environ]: SET_MESSAGE: ROM loading failed...
[ERROR] Failed to load content
Looks like it doesn't like your WAD. Try using another source. I've found one that works; I can't tell you where to get it but I can probably get away with sharing the crc32 and md5sum to help you determine if you've found the right one:
@sleve_mcdichael I'm not selling anything, it's your decision to use it or not. I'm not benefited in any way.
I'm just talking about my personal experience with it. And that, once everything is set up, it stops being a problem.
RetroPie doesn't have a Doom Legacy package - it's probably something extra included. I don't find any information about the RGB-Pi OS being based on RetroPie (though the documentation has some info that include RetroPie specific paths), nor a source repository for the image - maybe ask the image author about the issue ?