Greetings!
This is Tin, I'll be happy to assist you. Looks like you're unable to delete your old computer on the list of connected devices to your MS Account. Let me help you with that.
May I know if you're using a Company device or company account?
Have you tried to remove it from a browser?
Please try to delete it from a browser, please go to https://account.microsoft.com/devices and sign in your account. Please look for the device that you would like to be removed, click Show Details click More actions and Remove.
Then tick the "I'm ready to remove this device" and hit Remove.
For your reference, you may check the link below:
Thank you,
Tin