Faced the same issue yesterday. Tried from Mac and then from iPhone. Added a new payment method as well. Nothing worked. Contacted Apple Developer Support and they did something on their end. And 5 mins into the call, I tried again on developer website and it worked. I had to fill my personal details again and then, I was able to complete the payment. Got an email saying from Apple saying "Order is being processed". Customer care guy told me it would take 24h for developer account to become active.