can't erase mac

No, I mean are you using Mac #1 as your usual user and trying to erase Mac #1's drive. The one you are using is the active drive. If you have two Macs up and running, you have two active drives. On neither can you erase itself and you don't use one Mac to erase another.

Exactly how are you attempting to erase a drive? The exact steps.

Use Disk Utility to erase an Intel-based Mac - Apple Support. This is Apple's guidance. Of course, there are other ways to go about this depending on exactly what you final goal is. For instance getting a Mac ready to sell is a little different from just an erasure.

If you're trying to erase #1, you don't need #2 for anything, nothin, nada ... shut it down and set it aside. Where did you get instructions to use one Mac to erase another?

You can't erase the active drive you are using. Are you trying to do that?

I know about all this and when you click "learn more" (only other option is to cancel) you get instructions on resetting security. Which is good to know.

"Erase All Content and Settings"

That's an informational option.