The TopCount function looks at a 'set' and then takes your selected 'count' and applies it to this set for your selected 'measure'.
However a 'set' has to be enclosed in curly brackets (MDX syntax requirements)
Therefore if you change the code to the following hopefully it should now work.
TOPCOUNT( {[Dim Account Base].[Account Name].Members} , 10, [Measures].[Estimated Revenue])