If you're interested in downloading the Sonic Mania RSDK, here's what you need to do:
With the RSDK data set up, you can now use it to analyze and modify Sonic Mania game data. You can use tools like the RSDK Toolchain to: sonic mania data rsdk download
If you already own the game but are getting an error saying the data is missing: If you're interested in downloading the Sonic Mania