There's a problem with the Founders input site, there has been a lot of threads where players complaining about the same issue. the site has been that way for months, so its not just you.
Founders FAQ: http://www.callofduty.com/founder/faq
My Code is Valid, but Doesn't Work?
Each code is one time use. If you believe you are getting an incorrect error please contact Activision Customer Service.(http://www.activision.com/support)
Contacting support is the best way to go, they will investigate it and apply it if everything is good.
*** You must have Premium before you can apply a Founders Status to your Elite Account. You will get the error regardless.
Heres a post a found from support (ATVIAssist)
Did you manage to get your Founder code redeemed? If not, we have a couple things you can try.
Make sure that your premium status is activated, since as stated above, premium status can take some time to activate. Play a couple of multiplayer games to make sure your status has activated properly.
Before you try redeeming the code again, clear our your browser's cache and cookies. Here's instructions for that:
You can also try checking your status on your console app, and see if there's an inconsistency on your playercard there.
Let us know if this works for you!