Real-time voice or video communication between users on mobile devices or computers
Hello, I had this same issue and thanks to the chat with Zel at Microsoft, I was able to add a billing address (and thus reactivate the Skype credit).
The key was to change both the "Display language" and the "Preferred language". You access this by clicking the "Language" field under the "Profile info" section of the Microsoft Account page. Once both the Display Language and Preferred Languages were changed, I was able to add an address and set that as my billing address, allowing me to ultimately reactivate the Skype credit. Good luck!