I have tried to upgrade to the monthly Adobe Stock subscription and Adobe will not let me. It says there is an error when I try to do it. I went into my accounts and refreshed my credit card info and cancelled my single pay as you go account with Adobe Stock but still no luck in getting it to work.
Earlier today there was an issue on the Adobe site that prevented orders from going through. The problem has been resolved and you should be able to complete your upgrade now.