Even the original Japanese release of Yo-kai Watch 2 shipped with a bug that displayed "save data was not recognized" at first launch. Level-5 released a patch on the same day to fix this issue.
Dedicated fan servers often have channels dedicated to sharing save files, mods, and custom textures. Yo-kai Watch 2 Save File Download
Before restoring a new file, always use Checkpoint to back up your current progress. Even the original Japanese release of Yo-kai Watch
: Downloads typically feature maxed-out items, such as Mega Exporbs for leveling and rare Crank-a-kai coins. How to Use a Downloaded Save Before restoring a new file, always use Checkpoint
Step-by-Step Guide: How to Install a Yo-kai Watch 2 Save File
Some save editors claim to support Medallium editing, but users have reported mixed results. One user on GBAtemp asked "Is it impossible to edit the Medallium?" and received no definitive answer, suggesting this feature may be limited or unreliable.
Community hubs are the best place to find reliable "100% completion" or "New Game+" resources: