It may also be worth noting that one software may not be as good as another.
Whilst we may prefer different things, you can see by the attached image, that MSRT is reporting some infections and Windows Defender found no problems.
So, what should be used or not, I'm not too sure.
I prefer to use all that I can.
That's why I posted here, to see if we could find out why my system was stopping me from running the tool.
And now, all of a sudden, it's working!
By the way, MSRT has only ever found a problem when I've run a full scan.