I just got my subscription to work. Wait a day and see if you can get the subscribe button to be "Blue" again, instead of grayed out. But importantly, use a desktop browser and not your phone browser when you complete the process. Tesla tech support advised me to do so, and it worked the next time around after the Subscribe screen finally reset. Not sure why, but they may have an issue passing through credentials or the credit card from mobile browser.
PS - mine is a September '18 purchase and my free subscription ended this month.