This error normally indicates that Amazon have been unable to complete their account checks. Confirm the following three points:
1) A valid debit/credit card must be present in the billing console with a correct billing address (click edit to view).
2) Your registered card requires a balance of at least $1, as Amazon will make a small charge of $1 to ensure that the card is valid and not stolen. This is temporary and will not be taken from your balance (otherwise known as an authorisation charge).
3) When signing up for AWS, did you receive a call from Amazon to verify your phone number? If not, you'll need to get in touch with Amazon so that they can verify this manually.
If you're still having trouble, open a case with Amazon's Account and Billing Support and let them know that you're getting an 'OptInRequired' error on your account. They'll be happy to point you in the right direction!