Sonic Mania cannot be played on the PPSSPP emulator. It does not exist as a PSP game, and there is no legitimate "ISO" or "CSO" file for it.
Here is a detailed breakdown of why this is the case, the risks involved in trying to find such a file, and the actual ways you can play Sonic Mania on mobile.
To get Sonic Mania running in PPSSPP, you generally need to follow these steps: Download the Emulator: Get the latest version of PPSSPP from the Google Play Store Get the Port Files: Find the fan-made Sonic Mania PSP Alpha or "Decomp" files, which usually include an File Placement: Create a folder named SonicMania in your phone's internal storage under into this folder.
Use a Windows emulator like ExaGear or Mobox (complicated, performance varies) — but not recommended for most users.