@qanba
Like what BuZz said, you need to give us more information so we can help:
https://retropie.org.uk/forum/topic/3/read-this-first

Also, please read this document:
https://retropie.org.uk/docs/Dreamcast/#controls

You probably just need to edit your controller's configuration file for Reicast/Dreamcast.

@BuZz OMG. Thanks, BuZz. It worked.. I guess this was some information that I either overlooked or just didn't come across. You just saved me from a great deal of stress.

Thank you so much, BuZz... You are awesome. :)

@i8allUrPi

I really appreciate your time and effort.

Not a problem. I'm glad it all worked out.

@maximus76 the page on the wiki works. If it doesn't show up, you should keep trying. Make sure you have the latest non beta firmware. I have 3 8bitdo controllers all paired with those instructions.

@BuZz great :) thx and controller ordered :)

@mattrixk Have you tried the tilde key?