trying to wipe computer for sale, unable to erase APPLE HDD? help please

In addition to what BobTheFisherman already told you, here is what I do in Step #7 of What to do before you sell, give away, or trade in your Mac - Apple Support:

1. Shut down the Mac
2. Boot into Internet Recovery (Hold down Shift-Option-Command-R keys during boot). For more info about this, see https://support.apple.com/en-us/HT201314. Best to do this with a wired keyboard, wired mouse & wired ethernet
3. When startup is complete, select Disk Utility.
4. In Disk Utility, erase the hard drive. I prefer do a secure erase of the hard drive (called Security Option 2)
5. When the erase operation finishes, exit Disk Utility
6. Reinstall macOS

You can not erase the disk you are booted into.

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