Just a heads up to new Google AdSense publishers, if you are trying to enter your PIN (personal identification number) and it is not working, you are not alone.
A Google Groups thread reports dozens of publishers receiving the same errors:
We apologize for the inconvenience, but we are unable to process your request at this time. Our engineers have been notified of this problem and will work to resolve it.
AdSensePro confirms the issue and says they are working on fixing it as soon as possible:
Our engineers are aware that some people have been having trouble entering their PINs, and are working to investigate and resolve this issue. There's no need to write into AdSense support, as they won't be able to give you any updates yet, but I'll be sure to post updates here as more information becomes available.
Thanks, and sorry for the inconvenience, -AdSensePro
An AdSense PIN is required to get paid. After you verify your address by entering in the PIN that was mailed to you, you will likely not need to enter a new PIN in every again. But you cannot get paid without it anymore.
Forum discussion at Google Groups.
Update: This is now fixed.