Have a different issue not covered here? Check the GBATemp thread for Pokémon: Let’s Go, Pikachu! under the Switch subforum. Do not ask for direct download links; the community will help with technical errors only.
Which are you currently using?
NSP files must be installed directly to the Switch’s internal storage or system SD card using an installer tool (like Tinfoil, Awwish, or DBI) before they can be played. pokemon lets go pikachu switch nsp xci upd work