This can be a very frustrating problem to run into.
The most common cause for sign-ups failing is your bank or credit card provider not recognizing the charge for security purposes, assuming your card may have been lost or stolen. This is still very common for online purchases.
The best way to prevent this is to either contact your bank or card provider and tell them you were the one who initiated the charge and make sure you're signing up from a device you've made purchases from before.
The second most common cause is fairly simple which is not having enough funds available on your debit card. Note that the front end price may be available, but taxes vary depending on your country and state.
But what about the deal you were offered as part of a special promotion?
Once you click on a link to a special offer even if it's only available one time, a cookie is placed on your computer or device that ensures your spot is saved and can be redeemed at any time by visiting the sign up page again.