【Updated: 】
You can link your Nintendo Account from within Animal Crossing: Pocket Camp.
If you would like to link your Nintendo Account from the Settings screen via the in-game menu, please click here.
If you would like to link your Nintendo Account from the title screen, please click here.
■ How to Link your Nintendo Account from the In-Game Menu
① Tap the menu icon at the bottom right of the screen.

② Tap Settings.

③ Scroll to the bottom of the page.

④ Tap Link Nintendo Account.

⑤ Tap Create/Link Nintendo Account to proceed.

■ How to Link your Nintendo Account from the Title Screen
①Tap Link Nintendo Account at the bottom of the title screen.

②Tap Create/Link Nintendo Account to proceed.

If this did not resolve your issue, you can also refer to these articles.
- What is a Nintendo Account?
- I want to transfer my current save data to another smart device.
- I want to create a Nintendo Account.
- Can I change or unlink the Nintendo Account that I currently have linked?