reset to factory settings on IMAC 2017 21.5" Retina 4K. My IMAC does not have "Erase settings"

Fj2503 wrote:

Previous owner is 100 Yrs old and can no longer see or hear well. He has backed up his information on a separate hard drive disk, as per instructions on "what to do before your sell, give away or trade in your MAC". Do you have any other suggestions?

The seller needs to follow the linked instructions to reset the iMac, relinquish ownership, to release Activation Lock, and related. That will release the ownership, reset the system, and the rest of what is involved in a transfer.

Sorry but the iMac is tied to the original owner ID and only they can release access to someone else or erase it to give away.

Is that person able to add you as a Legacy Contact?

see > How to add a Legacy Contact for your Apple ID - Apple Support

If not, then read > How to request access to a deceased family member's Apple account - Apple Support

see > What to do before you sell, give away, or trade in your Mac - Apple Support

Which will require the previous owners help.