Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Hello !
The Q&A Assist answer is right : you cannot add a Mac device to your Microsoft account devices list. This list is only for Windows devices and Xbox consoles.
What are you trying to achieve exactly ? If you want to activate Microsoft 365 (Office apps) on your Mac, you do not need to register your device here. You need to open any Office app, and log in to your Microsoft account here.
You can learn more about Office activation here : https://support.microsoft.com/en-us/office/activate-office-for-mac-7f6646b1-bb14-422a-9ad4-a53410fcefb2
Please let me know if you have any other question !