Before we are able to create an article, we must ascertain that there is at least one entry for 'Tax Rate':
Create an entry along the lines of what you see below:
Besides the 'Tax Rate' we will need a 'Tax Group'. Please click on 'Tax Groups'.
Tip: The difference between 'Taxe Rate' and 'Tax Groups' lies in the fact that a 'Tax Group' may contain several 'Taxe Rates' and that one can specify for a 'Tax Group' if a delivery is subject to tax or not.