How can I erase my old mac’s hard drive

alison2 wrote:

The erase button is not activated in the disk utility to erase. It’s a mid 2012 Mac book and running os high Sierra.

You can not erase a drive that you are booted from...

How to erase an Intel-based Mac

https://support.apple.com/en-us/HT208496

Recovery: https://support.apple.com/en-us/HT204904

You can't erase the drive you are booted into.

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