Most ecommerce solutions don’t give tax details, to solve it,a) Define the tax at your engine merchant center, Google for example go to Settings > Tax and shipping. if you don’t charge tax, just select that option of not charging tax.
b) Secondly use the advance insert option on the 'Products' page of retailtower to insert the tax for the products. Use the sort option to group products before inserting if you have different tax for different product groups. This is the most recommended approach since the editing applies for other channels Eg. 2% can be inserted in the tax column of your products