tried to erase a mac drive got could not unmount disk

If you're using macOS Mojave or earlier, choose Apple menu  > System Preferences, click iCloud, then click Sign Out. If you're using macOS Catalina or later, choose Apple menu  > System Preferences, then click Apple ID. Select Overview in the sidebar, then click Sign Out. Learn more about deauthorizing computers used with your iTunes account.

Then enter your Apple ID and password and click Deauthorize. If you're using macOS Mojave or earlier, open iTunes. From the menu bar at the top of the screen or iTunes window, choose Account > Authorizations > Deauthorize This Computer. Sign out of iTunes in macOS Mojave or earlier Make sure that you have a current backup of your important files.

Moving to a new Mac? Before taking these steps, you can use Migration Assistant to move your files from the old Mac to your new Mac. What to do before you sell, give away, or trade in your Macīack up your data, then restore to factory settings by turning off certain features and services, erasing your Mac, and reinstalling macOS. Are you booted into RestoreMode with CMD+r keys on startup?

Tried to erase a mac drive got could not unmount disk