How to remove device that is no longer associated with your account?

Depending on what you are trying to do these articles will help you do it

View and remove your associated devices for purchases - Apple Support

Check your Apple ID device list to see where you're signed in - Apple Support

You can go to Settings > Name and the device will be shown at the bottom of the screen. Select the device you want to remove, and it will give you the option to remove the device. The attached link will help you out a lot.

Check your Apple ID device list to see where you're signed in - Apple Support