@SaulFabre , I'm not sure if that made sense or not, but the dependencies thing, you should do git clone --recursive to download those other repos. I always use that command when using git clone, even if there's no other repos needed to be downloaded. it's just a habit now, but a good one.
as for the error you get now, delete game from the root of that RSDKv5 repo. it's just a placeholder. then, go here and clone this repo:
https://github.com/Rubberduckycooly/Sonic-Mania-Decompilation this is needed for all sonic mania builds. copy the SonicMania folder from the root of that repo and put it in the RSDKv5's repo, on the root. now, rename the SonicMania folder to Game. try to build it again. you may want to make clean first, and delete the bin folder. the bin folder is where the finished dol will appear. it's created when you do your make command, so it's okay to delete it. I don't think make clean deletes it for some reason.