boot – Please remove the installation medium, then press ENTER – finishing Ubuntu installation in VirtualBox

I’ve just installed Ubuntu 23.04 in VirtualBox. Now the program says:

Please remove the installation medium, then press ENTER:

(please see the first screenshot.)

I’ve seen this post, this post and many others addressing exactly the same question, however, none of them have helped.

How am I supposed to remove the installation medium? What exactly does this mean? Also, where do I press ‘Enter’? Presumably inside the VirtualBox?

One website I came across says:

While the new operating system is rebooting, there will be a message saying “Please remove installation media and close the tray (if any) then press “ENTER”. To do this, click “Devices” at the top of the window and hover over “CD/DVD devices”. You should see a device with Ubuntu in the name with a checkmark next to it. Click this device to remove the checkmark, then press Enter.

When I try removing the checkmark (by clicking on it), the following message pops up (please see screenshot 2).

What should I do? Please explain as though you were teaching a monkey.
I do apologise if this post is far too basic for this SE; I hope it helps beginners like myself!