I recently tried making a level mod for Sonic Mania, but when I open it, it tells me to select the data folder, but there is no data folder, I try creating one and selecting that, it just tells me to select it again, even if I make the folder inside of the mod folder. I tried looking for tutorials, or solutions for this, but nothing showed up, all I could find was a person who just solved it himself and didn't say how he fixed it.
Did you extract the Data.rsdk correctly? If not, that's probably why. Another reason it could be failing is that the folders you're choosing either (A. Don't contain a GameConfig.bin file or (B. Don't contain the "Stage" folder. I think the folder needs both of those in order to function on Maniac.