This usually means the CIA file is corrupted. Redownload or re-dump the file and try again.
Navigate to the folder where you placed the CIA file (e.g., SD -> cias ). Select the file. pokemon ultra sun v1.2 cia
A file is the standard installation format used by the Nintendo 3DS operating system. Unlike .3DS or .CCI files, which act as standard game cartridges or ROMs, a CIA file behaves exactly like an application or digital download from the Nintendo eShop. This usually means the CIA file is corrupted