You can't use the same user data for Animal Crossing: Pocket Camp on multiple devices at the same time.
Please stop playing on devices other than the device you wish to play on, then try again.
If you aren't using the data simultaneously, please confirm that you are linked to the same Nintendo Account on each device. You can check which account you're currently linked to by following these instructions:
❶ From within the game, select More.
❷ Select Settings.
❸ Select Nintendo Account Management.
❹ Select Account Info Management.