Usually the reasons are: the lack of funds on your card or the lack of funds in case you’re trying to pay in a different currency (and the currency exchange fee of your bank is making the total payment on your end just a bit too high). Contacting your bank to clear off the international transfer attempt may also be needed (especially frequent for USA based people). Technical errors may be fixed by using incognito/private mode on your browser; disabling adblock might also help :) Sometimes just trying to re-apply the payment attempt later.
If the suggested solutions won’t work please contact our support here: http://support.awakenrealms.com/support/tickets/new