British English would normally say 'without prescription' rather than 'without a prescription'.
you have no right to buy that without a prescription, could be correct ?
You can't buy that without a prescription should be accepted.
Would that not be
Tu ne peux pas acheter ça sans ordonnance.
As opposed to taking about having a right, or being allowed to buy that.
"Can" is used to mean a) be able to and b) have permission to.
So, with the second meaning,
"You have not the right to buy that without a prescription" is a literal translation but is not accepted?
It's always harder when English is not your first language. Next time, try: "you don't have the right..."
Since "sans ordonnance" translates to without A prescription, why no French article for "ordonnance"