You did want to delete this account?
But you didn't want to save it? If you don't want to save the old account, then wait until macOS finishes creating the .dmg of the old account (this can take days depending on the amount of data in the account). Once the .dmg is completed, then you can just delete the .dmg file which will be located in the "/Users" folder. Click on the Finder's "Go" menu and type "/Users" without the double quotes to open a Finder window which should show you the .dmg file (along with your any other user accounts still on the system).