In 2011, I bought a used Mac Mini Server for $500. Knowing what I know now, the machine was failing and the guy unloaded it on a sucker. That was me. It worked for a while, but occasionally would spontaneously reboot. I could never get it to work properly the way I wanted it to. In June this year, it failed to boot. When I turned it on, it would give beep-beep-beep, five second pause, repeat the three beeps. Looking at Apple for the meaning of startup tones, this indicates that the memory is bad. When I ordered the replacement hard disk for my MacBook, I ordered some memory to test it. I got the smallest amount so I wasn't out much money. So 4 gig for $30.
I plugged the memory in, restarted it and got the same repeating three beeps. So if the memory is new and it's being reported as bad memory, then most probably the problem is further inside. I would guess that the motherboard would need replacing. I never had warranty on this Mac, so I would estimate it would cost about $500 to fix this machine. That's just not worth it. And after all the bad experiences I've had with the Mac Minis made since 2013, I will not buy another. I really don't trust them. I'm not going to throw good money after bad.
I'll gut this machine. It has two 2.5" hard disks I can use, and the memory will go elsewhere. Anything else is worthless to me. I'll trash it. I doubt if I will ever buy another Mac Mini, I distrust them so much.