We are using Stripe as our Payment provider. This means that you will be able to pay with MasterCard, Visa and American Express for the monthly and yearly subscirptions. The Lifetime subscription payment is handled by PayPal, which means that Discovery cards also are accepted. There can also be other reasons for declines.
If your credit card still isn't accepted, It might be that it is blocked for international payments or Internet payments. In these cases you need to contact your credit card issuer in order to un-block international/Internet payments.
If the above solution doesn't work, make sure that you are not over your limit or have unsufficient funds on your account.