Learning PREVIOUSMONTH

Hi!

I'm trying to be on top of the basics of DAX. One of my first challenges is to understand the PREVIOUSMONTH function.

When I read this page, it looked really easy. 

So I opened up Power BI and started to play around.

1. I created one date table 
2. Then I created one "Total Sales" Column 
3. And then I added one column to see "Total Sales Previous Month" 

My problem is that my "Total Sales Previous Month" is not showing anything.

I've read many posts about people having the same issue, but I've never found the root cause of why it happenned in my specific situation.

Any help is welcome, thank you !