If you're seeing this support code, please try the following.
■Steps:
- Make sure your Safari browser is not set to private browsing (private window).
- Open your device's Settings menu, tap Safari, tap Advanced at the bottom of the menu, and then turn off Block All Cookies.
- Open your device's Settings menu, tap Screen Time, Content & Privacy Restrictions, Content Restrictions, then set Web Content to Unrestricted.
- Open your device's Settings menu, tap Face ID & Passcode, then turn off Password AutoFill.
- If you're using the below apps or software related to data transmission, review and temporarily adjust your settings.
- Security software
- Content blockers
- VPN apps
- Turn off Low Data Mode by opening your device's Settings, then tap Cellular, Cellular Data Options, then Data Mode.
After you have checked these settings, try to log in using the below URL via the Safari browser, instead of from within the Super Mario Run app.
https://accounts.nintendo.com
If you are able to log in, try linking your save data again.
Note: Setting options may differ depending on OS version.