erase assistant not supported on iMac?

If you cannot use erase all content and settings, you'll need to erase your Mac in a different way, See this instructions on how to do that.

If the Erase All Content and Settings feature isn't available on your Mac, follow these steps instead. Before you begin, it helps to know which macOS your Mac is using.

1. Back up or transfer your files

If you don't have a current backup, you can back up your files to an external storage device or transfer your files to a new Mac.

2. Sign out of iTunes

If using macOS Catalina or later, you can skip this step.

1. From the menu bar in iTunes, choose Account > Authorizations > Deauthorize This Computer.
2. Enter your Apple Account (Apple ID) information and click Deauthorize. Learn more about deauthorizing.

3. Sign out of iCloud

When you follow these steps to sign out of iCloud, you're asked whether to keep a copy of your iCloud data on this Mac. It's faster to just keep a copy, because you will erase your Mac later. 

macOS Ventura or later:

1. Choose Apple menu  > System Settings.
2. Click your name in the sidebar of System Settings.
3. On the right, scroll down and click Sign Out.

macOS Monterey, macOS Big Sur, or macOS Catalina:

1. Choose Apple menu  > System Preferences.
2. Click Apple ID.
3. Select Overview in the sidebar.
4. Click Sign Out.

Earlier versions of macOS:

1. Choose Apple menu  > System Preferences.
2. Click iCloud.
3. Click Sign Out.

4. Sign out of iMessage

1. Open the Messages app.
2. From the menu bar in Messages, choose Messages > Settings (or Preferences).
3. Click iMessage.
4. Click Sign Out.

5. Unpair Bluetooth devices (optional)

If you do this on a Mac that doesn't have a built-in keyboard or trackpad, you will need to plug in a USB keyboard and mouse to complete the remaining steps. 

If your Mac is paired with a Bluetooth keyboard, mouse, or other device that you plan to keep, you can unpair it. This optional step prevents accidental input when the Mac and device have separate owners but remain in Bluetooth range.

1. Choose Apple menu  > System Settings (or System Preferences).
2. Click Bluetooth.
3. Move your pointer over the device that you want to unpair, then click the Disconnect button or remove button (x) next to the device name.

6. Erase your Mac

1. Shut down your Mac.
2. Start up from macOS Recovery, then use Disk Utility to erase your Mac. For details about both of these actions, follow these steps: Use Disk Utility to erase an Intel-based Mac.
3. When done, quit Disk Utility but don't restart your Mac.

7. Reinstall macOS

1. While still in macOS Recovery, choose Reinstall macOS, then follow the onscreen instructions. Learn more about reinstalling macOS from macOS Recovery. 
2. After reinstalling macOS, your Mac restarts to a setup assistant. Press Command-Q to shut down instead of continuing setup.

8. Reset NVRAM

1. If you're using an Intel-based Mac, follow the steps to reset NVRAM. This clears user settings from memory and restores certain security features that might have been altered.
2. After resetting NVRAM, your Mac starts up to the setup assistant again. To leave the Mac in an out-of-box state, press Command-Q to shut down instead of continuing setup.