Case 1: Quit any applications that may be using the disc, and then press the Media
Eject key (C) on your keyboard. If you have a problem to close the application, try this: How to force to close an application in Mac OS
Case 2: Open a Finder window and click the eject icon next to the disc icon in the
sidebar, or drag the disc icon from the desktop to the Trash.
Case 3: Log out from your user account by choosing Apple () > Log Out “User”, and then press the Media Eject key (C) on your keyboard.
Case 4: Restart your Mac while holding down the mouse button.