" How did this happened?

I don't have FBM Sku for this product."

Go into your inventory page and you are probably going to see that at some time you actually had clicked on the "I will ship" box because you DO have two offers -- one for FBA and one for FBM.

In your inventory ZERO OUT the FBM inventory and the problem should stop. If you don't want to ever sell it FBM you can DELETE the FBM offer by clicking on 'edit' in your inventory.