Second-hand MacBook is suddenly locked and is asking for a 6 digit PIN code to unlock

Update: I'm writing this for people who might be in a similair situation.

I went to the service centre where I've been told that it wasn't an MDM issue, but an EFI issue. Basically I had a firmware password set up on the machine and had no idea. I've never set it up before so it must have been set up by the previous owner. The weirdest thing was the fact that I my computer locked itself spontaneously while I was simply browsing the Internet. Why was it tiggered? Why did my computer ask me for the firmware password where I did not request any action that would require it? And why did I see a message at the bottom of the screen about the 'Find My' app on iCloud? I asked the service centre professional I was talking to why was the firmware password suddenly triggered and he shrugged his shoulders not knowing himself how.

Anyways, I was told that in order to lift the firmware password I had to provide my personal ID (passport, drivers license, etc.) and a number of the cheque of my purchase. Needless to say I do not have any cheque from the person got the computer from and have literally zero idea how to reach them. And even if I do, can I even be sure the person can provide me with the cheque or the original cheque of the purchase? Would my cheque from them even suffice? I have doubts for both of those questions. The person might be dead at this point for all I know (it's literally war here). I even tried looking through similair current advertisements of Macbooks on the website I have found the guy on but have not been able to match any sellers to my guy (I only have a tiny logo in a pictogram in my text conversation with him from a single screenshot from my old phone; l even tried to make an image search in Google based on that tiny logo to no results). Simply put - I cannot find the seller.

Can Apple do anything? No. I've been told by the service centre professional to contact Apple Support again to find if they have any workarounds. I did just that and have been told that there was no point in calling them since they can't do anything themselves and I have to go to the service centre again. I highly doubt it will be of any use since the professional directly told me several times that they can't do anything without a cheque number. A purchase confirmation basically.

Basically I have now a powered brick. A piece of metal with my childhood photos in it and my work that I cannot use. Thanks to Apple's obsession with privacy that affects users. You won't encounter similair issues with other electronics. Want to get a Windows computer from someone? Here you go! Want to buy a Macbook? Well, you should do it through certified Apple resellers or from Apple directly or otherwise you might one day find your 3,5 year old computer completely dead with no way to make it work again. I can't imagine this happening with any other big computer company. Apple are trully in the legue of their own with this one.

TLDR: Apple can't do anything, my Macbook is now an aluminum brick.

Advice: make sure you get an original cheque of the purchase or buy your Mac from a certified reseller. Or just don't buy Apple products full stop and have your mind at peace that nothing like that will happen to you. The annoying thing is I absolutely loved my computer and MacOS, but I guess now I have to wait a year or two or three to get enough money to buy another Macbook computer. This time - way overpriced to make sure it won't die on me in the future.

No, it’s a grey screen that shows up when the computer boots up after the Apple logo. I’m pretty sure I’ve reset the computer myself shortly after getting it. That being said, when I try to boot up the Mac through BootCamp Windows (by holding down ‘Option’) I do get a black screen with a padlock. The Internet tells me it’s a firmware lock related to the iCloud lock.

Thank you for your reply, I appreciate it.

The answer to the last question is yes, which I believe describes the apparent discrepancy. The previous owner(s) must absolutely, positively remove it from their Apple ID...

This is sad to hear... I've contacted Apple Support directly yesterday and they told me that it's potentially a MDM (Mobile Device Management) issue, meaning that the computer might have been tied to some company account, owned by a company, that now decided to lock it. Or lock all of their devices and mine just got into that bag as well. I'm seeing Apple Support directly soon hoping they will understand my situation. Problem is I don't have a receipt of the purchase which, unfortunately, will make this whole ordeal harder. I guess they can see that I am the only user and it is really me when they unlock the computer, but proving that I did in fact purchase it 3,5 years ago will be hard without a receipt. I'm hoping to somehow find the person I've purchased it from and ask them for a receipt now or any other proof, but that might be futile. I don't even remember where I've purchased it from and can't seem to find old messages from them (might have deleted them?). Even worse, I'm from Ukraine so the person might now even be doing their business anymore. Anyways, I appreciate the help.