WHen you see that error, check the menu Help->Show Errors. This should show you why the query did not work.
Error is "ORA-00936: missing expression"
It would be of tremendous help if you could post the failing query and what exactly you are doing. As for now the only thing we know is something is failing somewhere. Missing Expression means that your query isn't correct:
so from a missing keyword to missing object names this can be anything.