How to unlink Mac

Have a look at the steps mentioned in the related article:

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To disable Activation Lock, turn off Find My. 

1. From the Apple menu , choose System Settings.
2. Click your name in the sidebar, then click iCloud on the right.
3. Click the See All button (or Show More Apps button). 
4. Click Find My Mac, then click the button to turn it off. Enter your Apple Account password when prompted.

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If you can't turn off Find My

If you can't turn off Find My in iCloud settings, use one of these other methods to disable Activation Lock:

• Remove Activation Lock on the web.
• Use the Erase All Content and Settings feature to erase your Mac and reset it to factory settings.

If your previously owned Mac shows an Activation Lock window that asks for someone else's Apple Account, the previous owner may need to remove Activation Lock for you.

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copied from: Activation Lock for Mac - Apple Support