Trying to erase disk in iMac ... get message

You're welcome.

You bought it used or new from Apple?

If it is used, you may have trouble if the previous owner did not properly prepare it for resale.

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

If it is new, then you need to use macOS Recovery Mode to erase and reinstall macOS.

About macOS Recovery on Intel-based Mac computers - Apple Support

What year model iMac?

Identify your iMac model - Apple Support

You have to startup from a different disk. It can be the original Install DVD, an external startup disk or macOS Recovery Mode depending on the year model.

How to erase your Intel-based Mac - Apple Support