I am selling my MacBook air. disk utilities has greyed out erase ??

Additionally, make sure you follow all of these instructions:

https://support.apple.com/en-us/HT201065

And, as Encryptor mentioned, you need to boot into something other than your hard drive, preferably via recovery: you cannot erase the drive you are booted into.

Hi dougiptl,

If you're trying to erase your Mac, you'll need to start up from macOS Recovery. Follow these steps:

Optional: To protect your data, open System Preferences -> Security and Privacy -> FileVault. Turn on FileVault and wait for encryption to complete. This ensures that the previous owner can't recover your data. Then, follow the below steps, depending on what type of processor your Mac has:

Intel-based Macs: Restart your Mac while holding down Command-R. Release the keys when the Apple logo and a progress bar appear.

Apple Silicon-based Macs (M1): Shut down your Mac. Then, press and hold the power button until the screen says "Loading startup options". Click the Options gear, then click Continue.

Once in macOS Recovery, follow these steps to erase your Mac and reinstall macOS: How to erase your Intel-based Mac - Apple Support