2020 iMac - unable to install MacOS past Monterey 12.7.4

[Fishrrman]

I'm thinking that whoever sold this "cheap" to the OP...
... didn't actually OWN it.

And that the person who DID own it was the one who put the activation lock onto it...

 

[twr7cx]

Isn't this the problem though?
I haven't followed all of it but at the beginning you said 'Wasn't able to check and change the Startup Security Utility settings due to not having a macOS account details'.
...

I don't think so. As after I installed the early version of macOS on it and created an Administrator User Account in macOS, I was then able boot up and access Startup Security Utility settings using that accounts credentials that I had freshly created.

Perhaps related is that I never added my Apple ID account onto this Mac. Even when I created the new user account in the early versions of macOS, I didn't add my Apple ID details to it. I am curious if I had, if that may have overridden the Authentication Lock or replaced it with my details?
Who knows though? It all seems a big vague as to how it works and what triggers it. It's certainly not as linear and set process as the iPhone/iPads are.

 

[twr7cx]

That's true.. but System Information will display whether Activation Lock is enabled, regardless of local account logged in. See System Information.app > Hardware > Activation Lock Status.

Also, if you were to log into an Apple Account, you would not be able to enable Find My, as it is already enabled against a different Apple Account.

...

Now this is super handy information to know, that I was unable to find with my Google searches! Thank you very much!

I confirmed (on my other 2020 iMac) that System Information does show Activation Lock Status as you have described.
I also confirmed on that same other iMac, that Find My Mac is enabled in iCloud in System Settings.
So at least I know this iMac truely belongs to me!

But more important, this provides an avenue of checking and confirming the Macs status. So thank you very much!

 

[twr7cx]

I'm thinking that whoever sold this "cheap" to the OP...
... didn't actually OWN it.

And that the person who DID own it was the one who put the activation lock onto it...

Certainly possible. And given that I haven't had a response from them in the past four days it's seemingly more likely. Thankfully the auction house has a process for such issues that I'll lodge tomorrow.