[Solved]
Hello,
I followed the steps to the best of my ability.
I contacted apple as I got the familiar error message:
You cannot sign in to iMessage on this Mac at this time.
To use iMessage with this Mac, contact Apple Support and provide the code below.
Customer Code: xxxx-xxxx-xxxx
Apple ID: (my email address)
After contacting them and providing them with the code, the first device began to work as expected. However, this second device continued to give the same error message, even after at least a thousand different representatives across a hundred ranks tried to activate it on their end over the past week. I attached my Clover folder, as I want to make sure that stuff like the serial number, MLB, ROM, etc. are appropriate for iMessage.
This is happening on MacOS 10.15.6 Catalina. An Apple ID that has been used for this device in the past is working.
Update #1: I found out that my current MLB (Board Serial Number) has not changed since the days where my system was defined as MacPro3,1. Obviously, this is not good, as my system is now defined is iMac14,2. So I generated a brand new MLB corresponding with the iMac14,2 serial number using simpleMLB.sh and pasted it into Clover.
Now I have a different "Customer Code" or Verification Code, so I'm going to contact Apple again, telling them I did something along the lines of an SMC reset or firmware update.
On a side note, after I contacted Apple (before I posted today), my "invalid" Serial Number magically became valid in their system. Though there is no purchase date, warranty, etc., so I doubt my serial number is actually conflicting with a real Mac, let alone that it was invalid when I checked the first time.
Update #2: Even after changing the MLB and giving them the new code, it still didn't work. I think I "spoiled" the serial number. I just changed serial number, made new values for everything (ROM, MLB, SmUUID, etc.) and gave them the code for a third time. Works fine now.
Moral of the story? If you contact Apple and it still doesn't work after you give them the code, start COMPLETELY fresh (new SN, new MLB, new ROM, new SmUUID, etc.) before trying again (I also deleted iMessage files just in case).