I am trying to erase my hard drive. The "erase" button will not highlight for me when I select Macintosh HD. How do I get it to highlight so that I may proceed with the erasure.

You cannot erase the drive you are currently booted from. You will have to boot from another drive, or boot the Mac in recovery mode, launch Disk Utility and erase the drive from there. Then you'll have to reinstall macOS.

How to reinstall macOS - Apple Support

About macOS Recovery on Intel-based Mac Computers - Apple Support

Why do you feel it necessary to erase your drive? That is usually the last thing you want to do.

If you are having some issue with your Mac, then post about it here and let's see if we can find guidance that doesn't include wiping the drive.

Since you are selling your computer, please see the below article for properly preparing the Mac for transfer –

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

Please see the below articles for info about recovery mode in macOS. You may need a usb keyboard to get the Mac to recognize the following key combinations at startup:

To start up your Mac in recovery mode, press the power button. When you hear the startup chime, press and hold the Command-R keys (⌘R) until you see the Apple logo and the progress bar.

If Recovery mode doesn't work, maybe use Option-Command-R or Shift-Option-Command-R to boot from macOS over the internet.

Mac startup key combinations - Apple Support

Also, please see the below article for information on creating a bootable installer for a Mac –

How to create a bootable installer for macOS - Apple Support

Are you trying to erase the active. i.e. operating main disk? You can't erase that disk from within itself.

Can you explain how to "boot from another drive" or how to "boot the Mac in recovery mode"? I'm having the same issue. I'm wiping the drive because I'm selling the computer.