Since this is a fangame, it is distributed for free. You will not find it on the Google Play Store due to copyright restrictions (it uses official SEGA assets). You must download it directly from the GameJolt platform.
GameJolt is a popular platform for indie game developers, and Sonic.exe: Spirits of Hell Round 2 is one of the many games available on the site. To play the game on Android for free, follow these steps: gamejolt sonicexe spirits of hell round 2 android free