Disk Utility Question

I have followed all the items on What to do before you sell, give away, or trade in your Mac - Apple Support I am at the point of using Disk Utility to erase my disk, but "Erase" is greyed out. How to I proceed?

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

Hi Bill,

Disk Utility greys out the Erase option if you're trying to erase the disk you're started up from. This is done in order to avoid system corruption.

To erase your Mac, follow these steps:

1. Make sure you have a fast and stable Internet connection, and plenty of time.
2. Start up in macOS Recovery: Hold down Command-R as you restart your Mac, and release the keys when the Apple logo and a progress bar appears. You might see a spinning globe instead.
3. If you're prompted to enter your administrator password, do so to continue.
4. You should now see the macOS Utilities window. Select Disk Utility and click Continue.
5. Click View -> Show All Devices.
6. Select the top level of the internal drive (APPLE SSD/HDD) and click Erase. Choose these options, then click Erase:
   1. Name: Macintosh HD
   2. Format: Use the format pre-selected by Disk Utility. It should be either Mac OS Extended (Journaled) or APFS.
   3. Scheme: GUID Partition Table/Map
7. When done, quit Disk Utility, and select Install macOS.
8. Follow the onscreen instructions to start installing macOS.
9. When the macOS Setup Assistant appears, press Command-Q to shut down your Mac. It is now ready to be traded.