Your 3DS must be running modern CFW (Boot9strap and Luma3DS).

Popular ROM hacks (like Rising Sun or Supernova Sun ) often require the base game to be updated to v1.2 to prevent asset conflicts. How to Install the Update on a Modded 3DS

cias , choose the v1.2 update file, and select to save precious block space on your SD card.

Restored stability, though old v1.0/v1.1 videos become unplayable. Why the CIA Format is Essential for 3DS

Do you need help finding guides to ?

The version 1.2 update, released in early 2018, wasn't a mere bug fix. It was the game’s final competitive patch. It added support for the newly discovered Ultra Beasts (Blacephalon and Stakataka), tweaked move animations, and—most critically—implemented the data for the Zeraora event. Without v1.2, players cannot legitimately (or illegitimately) access the mythical thunder cat.

Having trouble with multiple Pokemon Ultra Sun CIAs : r/3dspiracy

Do you plan to play this on a or a PC emulator ?