Add "--menu --verbose >> retroarch.log 2>&1" to the command line parameter of Retroarch and run the game again from LB. It will generate a retroarch.log file in your retroarch directory. The log file might tell you what's wrong.
https://docs.libretro.com/guides/generating-retroarch-logs/
Also check, if the platform on which you have imported your mame roms is maintained under the associated platforms and is poiting to the right Retroarch core. For me the Platform is called Arcade, so my setting looks like this: