I can t find my internal HDD drive (part of fusion drive) on iMac 27 late 2012
It could be a hardware failure. You could test that by installing the OS in recovery mode to an external drive (USB/FireWire/Thunderbolt are all supported for booting) and then seeing if you can wipe both the HDD and SSD or set up Fusion Drive again.

This delays spending time and money on a repair tech. Or spending the time to decide if you can attempt a DIY repair or want to get it to a certified / trained tech that has access to Apple repair parts and tools.