if there an error on the lower right of the map, click on it.
It will open and pop menu with the detail of the assignement that Tableau has done.
Sometime, it happens that Tableau does not recognize the country of state and then, could not draw the map.
Also, as Tableau requires internet access to take geo information, make sure to have a internet access.
If you still have issue, I would recommend to give us the workbook to let us analyze the issue deeply.
I've now solved it - the problem was that the US was listed as the location, whereas I wanted the UK, which I have now changed. It seems to be working now.
Thanks for your help.
You're welcome :-)
Could you mark my answer as correct please ?