The relevant part of Article 1, Section 8 of the Constitution states that Congress has the sole authority "To regulate Commerce with...the Indian Tribes." The relevant part of Article 1, Section 10 states that "No State shall...lay any Imposts or Duties on Imports or Exports..."
Some states try to tax (an impost) the sale of products sold on Indian reservations to non-Indians. This violates the aforementioned sections of the Constitution because only Congress has the authority to regulate commerce with the tribes and states are not allowed to tax imports. (Since the reservations are effectively sovereign nations, products brought from them into a state are imports). Have a federal tax or duty on those products if you want, but the states have no authority to do so.