The violation of their constitutional rights as Englishmen was that they were being taxed for the sole purpose of raising money for the Crown, without having any say in the decision-making process.
The 1764 Sugar Act created new taxes on goods like sugar, coffee, and cloth brought into the colonies.
The English colonists in America viewed the Sugar Act as a tax which Parliament had no right to impose onto them, because they didn't have any representatives in Parliament and they were supposed to be represented and governed by colonial assemblies.