Hi milfara,
Considering that the MacBook Pro you bought was made in 2014, it doesn't have the T2 Security Chip or an Apple silicon SoC. Because of this, your Mac most likely got locked via Find My Mac by a previous owner.
Apple can remove this lock, but they will require thorough proof of ownership. Unfortunately, if Apple doesn't accept the documents you have regarding this, the lock cannot be removed.
As mentioned by HWTech, this is one of many dangers of buying used Macs online from a third-party marketplace. If you want to purchase a used Mac, the safest place to do so is in the Refurbished Products section on the online Apple Store. All of those devices have been put back in good order by Apple and come with the same guarantees as a brand new Mac.