Now I am going to provide a cliff notes version of resolving this error and will provide links to other resources for more details on the solution.
ERROR: A set has been encountered that cannot contain calculated members.
Now the quick and easy way to resolve this error...
- In the SSRS Solution Explorer double click the datasource to open the Data Source Properties box.
- In the text box for connection string: append the connection string with ;Subqueries=1
- Click OK
- Have fun using your Calculations.
Need a little more: Here is a like to Mike Milligan's Blog with some more detail and step by step.