Booting up to a no entry sign... OCLP?

I have a 2012 MBP 13" here that boots upo to a no entry sign, we have tested with 3 HDD Cables and swapped a known good SSD with a clean install of Catalina on it, to no avail. We can safely rule out a cable issue, SSD, HDD, or RAM as all are good. I cannot get into boot options or diagnostics either so I am thinking this may have been patched with OCLP before as the previous owner has kept the SSD from it and they don't respond to any questions asked (They don't know)..... If my assumption is incorrect, any ideas what we need to look into as I have tried just about every other option....?

If your Mac starts up to a circle with a line through it