how to unlink a Macbook pro from previous owners icloud
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MacBook Pro 13″, macOS 10.13
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Contact them and have them delete it from My Devices on Find My (they can access that in a browser or on an iDevice they still have). The whole point of Activation Lock is that someone who "finds" your Mac cannot just start using it.
Activation Lock and Find My on Mac - Apple Support
If you purchased the Mac from another person, they should have followed the steps here:
What to do before you sell, give away, or trade in your Mac
If they did not, contact the seller and have them do so.