erase hard drive

You need to boot to an online macOS installer using Internet Recovery Mode (Command + Option + R) or by using a bootable macOS USB installer.

I recommend creating the macOS USB installer for macOS 10.15 Catalina so that you have options if something goes wrong since Internet Recovery Mode can be problematic for some people.

What is your ultimate goal here? Are you trying to prepare the laptop for sale or disposal? Or are you trying to perform a clean install of macOS for yourself?

If you are preparing the laptop for disposal, then here is an Apple article on the steps you should follow:

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

FYI, if your laptop is using a hard drive, then you should modify the last step since Apple assumes the Mac is either using an SSD or that Filevault is enabled. I highly recommend you first enable Filevault and let the encryption process finish (may take a few days to finish depending on the speed & health of the drive). Then while booted to a macOS installer launch Disk Utility and erase the whole physical drive as GUID partition and MacOS Extended (Journaled). You may need to click "View" within Disk Utility and select "Show All Devices" so that the physical drive appears on the left pane of Disk Utility. The physical drive is usually listed by the make & model of the drive such as "Apple SSD...." or "Hitachi HT.....", etc. If you don't first encrypt the hard drive with Filevault, then you will need to write zeroes to the whole hard drive, but this is difficult since it requires use of the command line.