Hello Melanie J,
Welcome to Microsoft Q&A forum. It's my pleasure to help you today.
I understand that you are having trouble adding your device to your account.
Before we proceed, may I know more details of what happened when you try to add the device to your account? A photo would be best, please remember to remove all personal information before posting.
Just want to make sure, the changes made to an account might takes sometime to be fully reflected on the website. So please try to wait a bit and go back to account.microsoft.com/devices and check again.
I'm waiting for your reply so we can work on it together.