There are a couple of reasons you could be unable to purchase postage due to a credit card issue.
If you have not received a new credit card:
It may be best to contact your bank to ensure that they will allow the charges to go through. As postage is considered a form of currency, there are strict regulations around postage purchases so the banks can be quite strict.
If you have received a new credit card:
You will need to update the information with Endicia for both
your Endicia First Class and Endicia Expedited accounts. These are separately managed accounts, so it is important that your billing is updated with both. Find out how to update your billing information here