It is most unlikely that you will see this error message when trying to add credit to your meter, but if you do, it means that either the browser couldn’t create a secure cookie or access it.
This can be caused by ad- or script-blocking programs, but also by the browser itself, if it’s not allowed to set cookies.
The easiest way to solve this issue is simply to use another browser, eg change to Chrome, Safari, Firefox or Internet Explorer.