Why will an iMac 21.5" Late 2011 not accept an operating system?

Let’s do this!

From your working system connect the SSD externally using a SATA to USB adapter (or drive case via USB or TB) Reformat it to GUID Journaled file system (HFS+). Now install the SSD into the system in question. Now using a bootable USB OS installer with Sierra Not High Sierra! Install Sierra onto your system. It will update the systems firmware. Now if you really need High Sierra install that afterwards!

OK, why?? Your systems firmware was still running an older version that can’t be upgraded to High Sierra until you get the Sierra. This gets into the fact HS alters the file system of SSD’s to APFS (if you had installed a HDD the file system would have stayed at HFS+). So when you prepped the drive externally or in the other system the firmware was no updated with the version that understands APFS!

Now the other shoe drops! SATA based drives don’t run well with APFS! Without getting deep here. APFS is a very chatty dialog unlike what HFS+ uses. It was designed with PCIe/NVMe drives which have a wider I/O and many more buffers.

Let’s think this like an old fashioned fire brigade using buckets of water passed between people in a line. One line would pass the full buckets from thew ell to the fire and the buckets would be handed back so they are constantly crossing each other one full for one empty. How fast can you go? Likely a few buckets will be dropped both full and empty! Not very good ;-{ Now what if we had a second line of people who only handled the empty buckets then we would remove the conflict of the people dropping buckets! SATA is just like this one line which needs to handle both the movement and the Ack of the movement, so if we have a chatty dialog we get into the higher flows of Moves and Acks!

Of course we could even add more lines (PCIe lanes) and as PCIe has many more buffers can queue up things much better!

So bottomline: SATA based systems just work better with HFS+ on either HDD or SSD drives!

I do I like APFS! But on PCIe based systems, I would only use the Mojave version not High Sierra as Apple altered APFS between them.