You entered the code incorrectly.
Check for any typo's, check if you entered the 16 digit Nintendo code.
Copy the code which you received from us and go to the following website to redeem the code: https://ec.nintendo.com/redeem.
You or someone else already used this code.
Restart your device and re-login with your Nintendo ID, if the Nintendo credits are not activated in your account - contact Nintendo support.
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