Ok so looks like I had the mu-plugins folder in the wp-content/plugins folder when it needed to be in the wp-content folder.
I have moved it and get the following error:-
The domain mapping plugin only works if the site is installed in /. This is a limitation of how virtual servers work and is very difficult to work around.
I am running this on my localhost in www/foldername/ is this the reason it’s not working? Will it work online?