"Erase Assistant is not supported on this Mac"

Excerpt for suggests link at top of this question

If you can't use Erase All Content and Settings

Erase All Content and Settings is available only in macOS Monterey or later, and only on a Mac with Apple silicon or a Mac with the Apple T2 Security Chip. If this feature isn't available or doesn't work on your Mac:

• To reset your Mac to factory settings, which includes erasing your Mac, follow the steps in what to do before you sell, give away, or trade in your Mac.
• To only erase your Mac, learn how to erase a Mac with Apple silicon or erase an Intel-based Mac. 
• When using Erase All Content and Settings, if you see a message explaining that other volumes need to be erased before you can erase all content and settings, you might have used Boot Camp to install Microsoft Windows. In that case, use Boot Camp to remove Windows and its partition.

Is your Mac a T2 or Apple Silicon Mac?

The command should not be available if it is not.

There is a section at the bottom of the Related Article that discusses what to do if you can’t use that option.

What related article, please?

The article linked at the top of this page labeled: Related Article:

I can’t rest my Mac