Want to throw out old have old IMac 20" intel core duo. need to erase disk drive. Command R does not work

Please carefully follow the steps in What to do before you sell, give away, or trade in your Mac and after please use Apple's free recycling service. Apple Recycling Programs

Pop in the original gray Disc 1 that came with that iMac or a white Snow Leopard Upgrade DVD and restart holding the Alt (option) key. Then once your started up from the Disc or DVD, go up on the menu bar to Utilities select Disk Utility and erase the Macintosh HD.

The other option is to search for a DIY repair guide and physical remove the hard drive before recycling the iMac.

An inexpensive USB aftermarket LG will work with those old iMac’s and a new Mac or PC.

https://www.bestbuy.com/site/lg-8x-external-usb-2-0-double-layer-dvdrw-r-ram-cd-rw-drive-black/5581020.p?skuId=5581020

The other option is to open her up and remove the hard drive.