mdm key?

I believe you will find more information about MDM Key at this link: Activation Lock on Apple devices – Apple Support (UK)

Return the laptop for a full refund.

This laptop was not properly prepared for resale by the previous owner, or the laptop may be stolen. Only the previous owner who is managing the laptop can remove it from their MDM. This is just one of the dangers of buying a used Mac these days.

The other danger is whether the Mac has a firmware password lock enabled. You can check by holding down the Option key immediately after powering on the Mac (Intel Macs only). If you see a padlock, the Mac has a firmware password lock enabled...there is nothing you can do to remove it as it requires acceptable proof of purchase which you won't have since you are not the original owner.