I have been trying to set up a macro which will run when I click on a
cell, to change the value in another cell.

I have tried a number of variations around the following around Private
Sub Worksheet_SelectionChange(ByVal target As Range), but I'm getting
an error saying "Ambiguous name detected: Worksheet_SelectionChange".

All I want to do is run macro that checks the value of the clicked cell
and update another cell if appropriate.