Thank you for your prompt reply and for the screenshot, which was very helpful. The problem was, as had been suggested earlier, the fact that the DVD containing Solus was not mounted. which is very embarrassing. What I cannot understand, however, is that every other time I have installed an OS on VirtualBox, it has immediately recognised and mounted the DVD drive. I don't l know whether VirtualBox has changed something or I've just been lucky in the past.
I very much appreciate the help you've given me. Now to install both MATE and Budgies on VirtualBox .