The Erase button is grayed out. How do I erase my disc?

Have you in Disk Utility selected the device you want to erase?

You can not erase the drive you are booted into. Use recovery.

This article provides details:

How to erase a disk for Mac - Apple Support

You need to boot into Recovery Mode and run DU from there. Choose the uppermost, unindented icon representing the entire drive. (You may need to choose View in the upper left to reveal it)

About macOS Recovery - Apple Support