The registration page should display a decline message indicating the cause of the failure.
Check your card holder name, billing address and credit card details carefully for typos.
Try another funding source if necessary.
Some credit card companies apply holds to online purchases to protect your account security. You may need to consult your financial institution by phone to remove such a hold.
PayPal may decline a subscription agreement if your PayPal account is not linked to a renewable funding source such as a credit card and or the current balance is lower than the subscription rate.