can't erase computer with Mac os Big Sur

You MUST be booted from recovery or from an external drive in order to erase the internal startup drive.

If the computer fails to boot into recovery while using a wireless keyboard, you should try again with a wired USB keyboard. Very often a wireless keyboard cannot respond quickly enough to put the computer into recovery. Most any USB keyboard will work, even a PC kbd.

NOTE: When using a Windows keyboard on a Mac, the Windows key is used in place of the Mac’s Command (⌘) key, and the Alt key substitutes for the Mac’s Option (⌥) key.

Use macOS Recovery on an Intel-based Mac - Apple Support

1. ❝On your Mac, choose Apple menu  > Restart.
2. Immediately press and hold one of the following key combinations until you see the startup screen:

• • Command-R: Start up from the built-in macOS Recovery System. Use this key combination to reinstall the latest macOS that was installed on your system, or to use the other apps in macOS Recovery.
  • Option-Command-R: Start up from macOS Recovery over the internet. Use this key combination to reinstall macOS and upgrade to the latest version of macOS that’s compatible with your Mac.
  • Option-Shift-Command-R: Start up from macOS Recovery over the internet. Use this key combination to reinstall the version of macOS that came with your Mac or the closest version that’s still available.❞

Click here and follow the instructions; in particular, if you don't have the option for that in System Preferences, the computer needs to be running from the recovery system to erase the internal drive.

(257653)

For an Intel Mac, select the physical internal drive to erase. Within Disk Utility you need to click "View" and select "Show All Devices" before the physical drive appears on the left pane of Disk Utility. Erase the physical drive as GUID partition and APFS (top option).

Why Erase ,and Donate this iMac 24",Computer?

Linux is a possibility?