Startup Security Utility, no administrator found, The Recovery Server Could Not Be Contacted.

Am guessing it's a T2 MBP... If the Base system partition of the said machine is okay... delete the Macintosh partition, get an external drive APFS formatted and Restore the Macintosh drive of the same working MBP onto the drive... then restore the macOS Image on the external hard drive onto the "no administrator found" MacBook... in Recovery mode the machine will allow you restore IMAGE but can't boot from external drive. Good luck.

User wrote " I bought a 2018 MacBook Pro on eBay, at first it had MDM lock, which the seller disabled,

Return the computer to the Seller for a Full Refund asap.

The MDM Service has the Serial Number of this Device Registered in the MDM Service

As long as the Serial Number is registered with them, they can at any time Put the Lock Back on the computer

Even Wiping the Drive 100% Clean of everything and installing a new copy of Ventura will Not Remove the Serial Number for the MDM Service Database

You should have used your bootable macOS USB installer and erased the internal SSD while booted from USB. The 2018-2020 Intel Macs utilize a T2 security chip which requires at least one valid admin user account to be registered on the computer which also means the internal SSD must be fully functional including a working macOS. Once you erased the internal SSD, you also destroyed any known macOS user accounts associated with the internal SSD boot drive.

As @Barney-15E already mentions, your only option now is to reinstall macOS from Recovery Mode (Command + R) or Internet Recovery Mode (Command + Option + R) which should allow you to reinstall the last installed OS or to install the latest supported OS for that system (both will be installed from online servers through Internet Recovery Mode since the internal SSD is now blank).

If you have any issues with Internet Recovery Mode, then you may need to perform a firmware "Restore" which requires access to another Mac running macOS 12.4+.

Revive or restore an Intel-based Mac using Apple Configurator - Apple Support

If you receive large negative error codes when trying to boot into Internet Recovery Mode, then try connecting the Mac to the router using an Ethernet cable for a faster & more reliable network connection which bypasses hard to troubleshoot WiFi issues (you would need a USB-C to Gigabit Ethernet Adapter as well).

Apple has made a large number of changes to their Macs over the last several years, so I highly recommend you do some research to better understand how these newer Macs have changed & how they work now. A lot of the old tricks no longer apply (or are no longer reliable).

The only way to solve this would be to somehow get Internet Recovery to work and allow you to reinstall the OS on a blank drive.

The seller should have done all of this before selling it.

Did you get a Startup Assistant when you opened it the first time or did you just log into a previously created account? Since it had MDM, even if you went through the Startup Assistant, it likely didn't create your account as admin capable.

It sounds like there was never an admin account so you cannot reset the Startup Security.

It would probably be best to try to return it for a full refund as they failed to follow Apple's guidelines an preparing it for sale.

In his case, he can't boot from an external drive... "no administrator found" disables his ability to do that... Reinstall macOS in recovery modes via the Apple server and find paid ways online to remove MDM moving forward or you'll have a brick on your hands.

law-macbookpro wrote:

In his case, he can't boot from an external drive... "no administrator found" disables his ability to do that... Reinstall macOS in recovery modes via the Apple server and find paid ways online to remove MDM moving forward or you'll have a brick on your hands.

I hope the "law" part of your name isn't because you are somehow working in a law profession.

HWTech didn't tell him to boot from an external. It was a hypothetical on what should have been done from the beginning. Following that statement of warning to others, HWTech explained the only options left are Internet Recovery. Everything you stated was explained in detail by HWTech.

If it still had MDM activited, installing from Internet would have shown that fact with a dialog stating it was being managed by some company.

You most likely have MDM on your machine

NO... 😂 "Law" in my name is just a name brother.