i cannot get my old mac to reset to factory so i can turn it in please advise

What is the exact model of this "old" Mac? What version of OS X or macOS is it running? At what point did it fail to be reset?

Did you follow the steps in the following support article?

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

Turn on your Mac, then immediately press and hold these two keys until you see an Apple logo or other image: Command (⌘) and R - from https://support.apple.com/en-us/HT208496

I have a late model 27” iMac 2013 running OS Catalina version 10.15.7

I turn it on, press keys Option, Command, R, P and hold for 20 sec.

The Apple logos comes on and begins to show it is starting up.

I am holding the four keys down and all my files show up on the screen.

I have done this more than once and the scene repeats itself.

I already purchased and am using our new 24” iMac and do not want anything transferred to it from my old computer.

I transferred all my files from old computer to an 5TB external drive.

I just want to erase all files on old computer and set it to original factory setting.