COUNTD() is only available in an 'extracts', not in 'live' connections.
Thanks for info. Is there any specific reason why Tableau Dev. Team is not providing the functionality in live connection?
It's a limitation of the MS Jet Engine. Microsoft didn't include the function. But when you do an extract then you're using Tableau database engine and they felt it was important to include COUNTD(). There are a few other functions that fall in this category, but at the moment I can't remember what they are.