Unable To Insert VBoxGuestAdditions.iso in my VM

I have Ubuntu 18 guest running on Windows 10 Host. I am trying to make my display full screen and on multiple monitors properly, so I am trying to get the VBoxGuestAdditions to make it happen, but whenever I go to "Devices -> "Insert Guest Additions CD Image" I get the following error message:

Unable to insert the virtual optical disk C:\Program Files\Oracle\VirtualBox\VBoxGuestAdditions.iso into the machine <name of virtual machine>.



Could not mount the media/drive 'C:\Program Files\Oracle\VirtualBox\VBoxGuestAdditions.iso' (VERR_PDM_MEDIA_LOCKED).

Result Code: E_FAIL (0x80004005)
Component: ConsoleWrap
Interface: IConsole {872da645-4a9b-1727-bee2-5585105b9eed}
Callee: IMachine {5047460a-265d-4538-b23e-ddba5fb84976}