Unable to insert the virtual optical disk (VBoxguestaddition) in virtual machine for ubuntu server in win 10

I am new in this kind of environment and I want to learn this. This is started once I install VirtualBox-5.2.18-124319-Win and successfully installing ubuntu-18.04.1-live-server-amd64 in that virtual machine, however in the instruction command me to install VBoxguestaddition, and I get this kind of error, eventhough I have 2 installations of VBoxguestaddition and VBoxGuestAdditions_4.3.20 in .iso;

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 {85cd948e-a71f-4289-281e-0ca7ad48cd89}

I tried use sudo command in terminal sudo mount /dev/cdrom/media/cdrom and not succeed, I ask my friend and lecturer but no one give a proper advice. Thank you in advance.

Fooling around with this problem, I clicked on Devices>Optical Drives and unchecked VBoxGuestAdditions.iso.

I then got a question asking: Would you like to try to force ejection of this disk? I clicked the [Force Unmount] box.

Next, go back to Devices>Optical Drives and add a check tp the VBoxGuestAdditions.iso

At least I'm no longer getting an error. Hope this works for you

I went to

Devices -> Optical Drives

and selected "Remove Disk From Virtual Drive". Then "Force Unmount".
Then I was able to select

Devices -> Install Guest Additions CD Image...

I just started using Ubuntu and came across this problem. When you get this error, it will show that the file(with its path) could not be loaded. You have a good look at it and maybe write it down somewhere.Now, you should look at the bottom right of your virtual machine window, you will see a CD Icon thingy. Right click on it and select "Choose a disk file". Click there and navigate with that path to the iso file. Click on it and a window will pop up, select 'Force Unmount'(weird thing the file infact loads). Now, right click on CD Icon again and you should see VBox something something.iso , click on it and a window will popup in Ubuntu, click run and enter your password. Done