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

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.

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