This happened to me too but instead of asking me to re-try using the code, they told me they cannot refund my points. Just submit another support request to MS reward support, make sure to copy the message that you get from Blizzard support in the MS reward support details/description section.
We appreciate your patience as Blizzard has been investigating solutions with our partners at Microsoft over the last week, and we’ve just received an update with instructions. While Blizzard cannot grant benefits for these codes, due to the way they work, and recent changes to Diablo IV’s Battle Plan, Microsoft’s support team can help reimburse the points spent.
The next step is for the accountholder to contact Microsoft using their support form:
rewards.microsoft.com/supportThe Microsoft support team will confirm the original reward code purchased/issued, and provide a reimbursement. If you are not the Microsoft Rewards user, and received the code as a gift from a friend, please have your friend contact Microsoft for assistance while logged in to their account.
I got my points refunded after my 2nd attempt. Good luck.