Why was my code refused?
The message says why: the code may have ended, reached its limit, already been redeemed on your account, or be for a different type of account.
From the guide Promo Codes & Account Credit.
Redeeming a code
Open Offers & rewards — on the website, the iPhone or the Android app — type the code under Promo code and choose Apply. The confirmation says exactly what the code gave your account. Codes are not case sensitive, and each can be redeemed once per account.
Read the whole guide: Promo Codes & Account Credit