Hi @qnfrr12 ,
I tried my best to reproduce this issue. As you said, this problem did happen to my personal Microsoft account on both Edge and Chrome. When I tried to add a security key, it prompted the same error. But when I tried to set up again, it worked somehow. You may try again to see if your security key can be added.
In short, this can now be regarded as an account-related problem. Unfortunately, for more information of personal Microsoft account problems, currently Microsoft Q&A does not have the support. If you need community support for it, you can go to Microsoft Community forum. Thank you for your understanding.
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Best Regards,
Shijie Li