Facing error while working with Aggregate function in MDX (MultiDimensional-Expresson)
Look again at the details for the Aggregate() function. You got the parameters correct, but you need to look at what the function will return. It returns a numeric value.

So your MDX evaluates as SELECT some-numeric-value ON 0 FROM [Cube] which is not valid. 

You need SELECT some-sort-of-set ON 0 FROM [Cube]

Try googling for examples of using this function (or see link above, which includes examples) to see how it can be used.