iMac (2017): Disk Utility not finding Mac HD to erase

Boot your Intel-based Mac in recovery mode.

How to start up from macOS Recovery - Apple Support

Launch Disk Utility, click View > Show All Devices, and then select the top-most drive media device listed below the Internal heading in the side bar. If it's the original drive it should be named APPLE HDD...Media. That is what you must select to erase and reformat.

Use Disk Utility to erase an Intel-based Mac - Apple Support

Describe the steps that led you to that observation. Start at the beginning, better yet start with the Mac powered down.

Then...?

If you want to erase that iMac's internal storage device in preparation for selling it, here are the instructions: What to do before you sell, give away, trade in, or recycle your Mac.