You should never download data.rsdk from a public file host unless you own a legal copy of the game.
The data.rsdk file for Sonic Mania Plus is from the standard version. The Plus version includes additional assets, character code, and Encore Mode level palettes. You cannot simply rename a standard data.rsdk to Plus and expect it to work. sonic mania plus data.rsdk download
To obtain the data.rsdk file correctly, you must own a legitimate copy of the game. Below is the guide on how to locate, extract, and use this critical file for your custom setup. How to Obtain the Legal data.rsdk File You should never download data