Apple has now hidden/deleted information about defective iMac 3TB Fusion drives in that timeframe that required a special replacement program. You might want to start there.I'm trying to reinstall macOS on an iMac (27-inch, Late 2013), prior to resale. I've already wiped the 3TB Fusion drive. I've tried various versions of macOS, both from USB flash installers and via Internet Recovery, but they always fail with an error such this one from High Sierra...
Otherwise, since High Sierra doesn't support APFS on Fusion drives, it seems like a scenario that may be prone to problems. Here's what I suggest if you sort out the question of defects above:
- With the internal storage "defused", check for SMART errors on both SSD and hard drive with a utility such as DriveDX.
- If you're sure there are no defects or hardware problems, boot a normal macOS Sierra 10.12.6 system and format the Fusion drive as HFS+, then try installing macOS Sierra. (Alternatively, you could try using macOS 10.14.3 Mojave instead, but I would use Sierra.)
- If that doesn't work for any reason, defuse the SSD and hard drive, format the hard drive in HFS+ format, and install macOS Sierra on that.