Earlier today, Riot's store had some issues with Prepaid RP cards like the ones you have. Redemption is currently bugged and I'm sure if you head over to the Support section, you're not the only one with these issues. I suggest you sit tight and relax, and let Riot fix it :D. Also, it wouldn't hurt to open a thread and to send an email just to say, what's up riot? :P
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-Shinussiel